I think they are pushing for a cap because of their desire to expand. Las Vegas is going to make zero TV money, but supposedly oodles of new gambling/touristy/unknown revenue streams (I'm very skeptical). Meanwhile they want to expand into two more cities, which barely makes any sense because they'd be expanding into markets smaller than the current markets that are complaining (bogusly) that their profit margins aren't big enough any longer. BUT the two new markets will certainly see a huge spike in both TV and stadium/auxiliary income in the short and mid terms. On top of that, they'll be paying a significant expansion fee for the other owners to swim in.
So I'm guessing the big market owners are now amenable to significant revenue sharing since they'll also get an immediate influx of new money from these new locations. Likely not as much to offset what they send back to the smaller markets, but it may be close enough where they can FINALLY see the long term health of the sport, in the form of parity, will help them profit more in the long run.
Nothing else makes sense to me. The big market teams that try to win have NEVER wanted a cap, and now they do? They obviously know there can't be a cap without a floor within 90% of it and likely giving players somewhere between 48-55% of the gross revenue; there's no way they are successfully modeling it after anything other than what the other three major sports do. Their angle is money, as always, but if the MLBPA plays their cards right, they should come out better than ever since this cap has always been looked at as a Scarlet Letter. It's symbolically bad for the top players, but practically good for 90% of the players.