I think it's because the league is working too hard on making sure that there is geographic parody. While the MLS has of course grown considerably, and can (should?) grow considerably more, we are not yet at the point where we can devolve into a European format, where at best 6-8 teams are so far ahead the rest of the league both talent wise and financially. We don't yet have the national brands to support that. (Fun question: who's the most "national" MLS brand? I'd say LA, Sounders, and Red Bulls.) Therefore, the salary cap is in place to try and keep everyone competitive. That way, regional interest skyrockets.
That's not it at all. And your post might have been really timely in 2006-2010 maybe. It's really ironic, in the process of pursuing "parity" they are essentially laying the seeds of disparity. Right now the cap is 3.4 Million a year. 1 DP can take a max of $475k out of that and the rest of the DP money comes out of club funds.
Basically the league parses out $3.4 million a year in monopoly money.... the players, other than DPs are paid by MLS. They basically work for MLS. So the problem is that you have small markets, mid markets and large markets.
Toronto, NYC, LA and Seattle and maybe on other are the only "large" markets teams that act like large markets. Red Bulls have gone into cost containment mode, or they are transitioning to a team that actually develops players versus buying DPs.
Then you have duds like FC Dallas, Chicago, Philly, and Colorado. Chicago and Philly just don't have deep pocketed owners. But the Hunts (KC Chiefs) own Dallas and they are tightwads, and Stan Kroenke who owns Arsenal and like 30 other pro sports teams is a tightwad as well.
You also have clubs like Kansas City and Portland, the very definition of mid market. They have full rocking stadiums for every game, large fanbases, and wealthy owners.
These are the types of teams that are really hurt the most by the DP rule and the cap. KC and Portland will never attract a Gerrard, Lampard, Beckham or Gio Dos Santos. They might get a Giovinco. But what they could get are some young pros that can command a couple of million a year. BUT, they can only sign three of them. Whereas LA, Toronto or NYC can easily attract three players that can command 5-7 million a piece.
The problem is that the rule structure and the cap has obviously outlived it's usefulness. Every team gets their 3.4 mil in allocation money, but the majority of teams are very interested in spending 5-20 million a year of their own money on players.
If LA can spend 20 million a year on 3 players then why can't KC or Portland spend 12 million on 6 players? There are so few players that are worth 5-7 million/year in wages that would ever play in KC or Portland.
Lastly, the reason why the costs are limited is because cheapskates like Robert Kraft, Clark Hunt, Stan Kroenke, Andrew Hauptman still want to run teams on the cheap and they have a disproportionate say in how things go.
In the latest CBA talks, there was a group of teams that pushed for greater flexibility in spending. NYC, Seattle, KC, LA and couple others pushed for it, but in the end they deferred to Kraft/Hunt group. I think that in five years time that won't be the case.