A previous version of this story was published in December 2018, and updated on Feb 7. 2019.
Aside from Bryce Harper and Manny Machado, the biggest name coming up in rumors this winter has consistently been catcher J.T. Realmuto, and finally, finally, a deal is done. Realmuto was traded from Miami to Philadelphia for a package of three prospects and international bonus slot money, sources told MLB.com on Thursday after months of rumors and discussions.
It's possible that fans outside of Miami don't really know how good Realmuto is, since he's never received a Most Valuable Player Award vote or driven in even 80 runs. But it's not hyperbole to say that he's the best all-around catcher in the game, in part because the catching heroes of recent years like Buster Posey, Russell Martin and Yadier Molina have begun to age, in part because young up-and-comers Gary Sánchez and Willson Contreras struggled in 2018 and in part because the offensive state of catching is historically weak right now.
The reason that so many teams were interested in acquiring Realmuto is because so few teams actually have an all-around catcher who can add value both next to the plate and behind it. Just look at how many catching leaderboards he was at or near the top of in 2018:
• First in pop time, 1.90 seconds
• Second in catcher arm strength, 87.8 mph
• Sixth in caught-stealing percentage, 38 percent (minimum 30 attempts)
• First in catcher Sprint Speed, 28.6 feet per second
• Second in slugging percentage, .484, of 27 catchers with 300 plate appearances
• Second in catcher wRC+, 126, of those 27 catchers
• First in catcher WAR, both with framing (5.8) and without (4.8)
On the hitting side, he did all of that despite having to deal with perhaps the biggest home-field disadvantage in the game. In his career, Realmuto has a .291 wOBA at home and a .364 wOBA on the road -- the largest gap of any regular hitter dating back to 2002.

You can argue about whether Realmuto is the best or the second-best or the fourth-best catcher if you like, but the point is that he's elite, which is why he was in extremely high demand, and this is where we get to talking about how poorly catchers across the Majors hit in 2018. (Spoiler alert: very.) 
To express that, let's look back at the past 100 seasons of baseball and look at how well catchers hit each year. We'll use Weighted On-Base Average, or wOBA, which is very similar to traditional on-base percentage, except that it gives more credit for extra-base hits rather than giving equal credit for each time on base.  
In that entire century of baseball, only three times have catchers hit worse than the .232/.304/.372 (.296 wOBA) they combined for in 2018 -- and all three times were in the low-offense 1960s, an era so devoid of production that the sport had to lower the mound in 1969 to account for it.
Weakest catcher hitting seasons, 1919-2018, wOBA
.281 -- 1967
.285 -- 1968
.290 -- 1965
.296 -- 2018 (tied with 1989 and 2015)

For context, the entire sport of backstops hit basically like White Sox infielder Yolmer Sánchez, who put up a .242/.306/.372 line this year. (If we look at it as a comparison against the league hitting average that year rather than as a raw number, it's still one of the 10 weakest seasons.)
What that means is that while Realmuto would be valuable in any year, he might be especially so right now, just because so many teams could use an offensive boost behind the plate. When the supply is low, the demand for one of the few all-around catchers is higher.
Now, the obvious question here is "why?" Why is catching offense down so markedly right now? There's not one clear answer, but a few theories include...
1. Maybe it's just a down year. 
Are we just in between catcher generations? We ran into this a few years ago at shortstop where there was a brief down period between the Derek Jeter/Hanley Ramirez/Jimmy Rollins era and the current standout Francisco Lindor/Corey Seager/Carlos Correa/Xander Bogaerts group. 
Behind the plate, older stars like Molina, Martin, Posey, Brian McCann, Jonathan Lucroy and Matt Wieters are generally not the same players they were a few years ago. Throw in surprisingly poor years from younger catchers like Sanchez, Contreras, and Austin Barnes, and maybe it's the perfect storm -- or at least it would be, if 2018's .296 wOBA weren't tied with what we saw in '15.

2. Maybe it's just harder than ever to be a catcher.
We talk a lot about how much more difficult it is to hit these days, thanks to the increase in velocity, breaking balls, and ever-increasing numbers of relief pitchers all showing different looks and repertoires. What if that's impacting catchers, too? We saw Grandal's high-profile struggles this October, but that's been an issue for Sanchez as well, and as FanGraphs showed recently, the past several seasons have had the highest rate of passed balls and wild pitches we've seen in years.
Because of all that, and because games are longer (in terms of pitches and time) than ever, and because teams are selecting for pitch-framing skill more than ever before, different types of players make it to the bigs behind the plate. It's harder to see sluggers of years past, like Mickey Tettleton or Mike Piazza, or even more recent names like Evan Gattis, sticking at catcher.
3. Maybe it's just harder than ever to develop catchers.
Catchers notoriously develop later than other positions, but what we're seeing now is something else. Five years ago, there were six catchers 25-or-under who had at least 200 plate appearances and league-average or better hitting. This year, there were none, one of only 14 times that's happened in the past 100 seasons.
Top hitting prospects like Harper and Wil Myers, who were amateur catchers, don't often get to stay there in the pros, as teams hope to expedite their arrival and limit injury risk. Kyle Schwarber's catching career is over; so, after some time behind the plate, were those of Joe Mauer and Carlos Santana.
If you go back to the 2011 MLB Pipeline Top 10 catchers list, only Yasmani Grandal has had an extended period as a quality starter; Travis d'Arnaud, Sanchez, Wilin Rosario and Derek Norris have had their moments. From the 2014 list, it's more of the same. Austin Hedges, Jorge Alfaro and Kevin Plawecki have been OK at best; Schwarber no longer catches and Blake Swihart barely does; Max Pentecost, Andrew Susac, Reese McGuire and Christian Bethancourt seem unlikely to break through.
The point being, it's harder than ever to catch, or to find a good catcher still in his 20s, so that's why Realmuto should cost a ton and will be worth it, which is why you could envision him on so many different teams. You see why the Mets prefer him to d'Arnaud or Plawecki, or why the Dodgers view him as a Grandal replacement, or how he would be a phemonenal fit in Houston, where Martín Maldonado and McCann are both departing the 2018 roster. (No, signing Robinson Chirinos doesn't prevent this move.)
Realmuto is valuable because he's a great player, the best at his position in the game. But he's valuable because he's a great player at the right time, when far too many teams are dealing with a total lack of offense behind the plate. It's hard to find a player who can hit well and receive, too. It's hard to find a Realmuto. It's why so many teams were willing to pay such a high price for him.