Mauer would've been a worthy choice, but it was clearly three options for two spots. There's plenty of justification for the choices they made that don't rely on the old, tired, poor-me coastal bias rationale (remind me again which coast the booming metropolis of St Louis is on)
For a list like this that evaluates players at a specific position and not as overall players, it matters that Mauer wound up playing slightly less than half of his games at catcher. That doesn't look great stacked up against Posey (83%) and Molina (97%). When determining the best catcher, that matters in a close race.
Also, right or wrong, team success is given a lot of weight in player legacy discussions, whether they're for a list like this, hall of fame candidacies, number retirements, etc. It generally bothers me how much weight is given to team accomplishments in an individual player evaluation (see: Manning, Eli), but it does have to be considered. When your competitors each won multiple championships in their primes while you never won a playoff game, that's going to affect the final tally, especially when you're a key contributor. Posey didn't exactly light it up during the playoffs, but he did have the highest bWAR on two championship teams and was third on the team as a rookie for the third championship team. And while Molina had a pretty mediocre playoff batting record overall himself, he hit well in their two World Series runs (especially 2006). So while I think Mauer gets waaaaaaaay too much heat locally for his teams' lack of playoff success (he won the same number of playoff games as a Twin as Rod Carew, after all), it does have to be acknowledged as a factor.
Mauer was worthy of a slot on the team. If I were picking, I'd pick Posey and Mauer (for all the talk about Molina's defense, it's weird how it's just completely ignored that he was a below-average career hitter). But his omission isn't egregious, and certainly doesn't require falling back on our collective regional inferiority complex