There are so many. Some highlights though:

• There’s a salary cap of $6.5M, which is actually more “League One” than Championship, but there are loopholes to exploit (Beckham rule and its offspring most predominant among them), and MLS is full of Americans and Western Hemisphere players who are good but would never get UK work permits so their wages are a bit depressed compared to second and third tier British players.
• Maximum senior roster of 30 players, of which 10 are (nominally) supposed to be on the equivalent of 1800 pounds/week. Exceptions here as well, but in broad strikes the bottom of the roster is WAAAY cheaper than even the middle.
• Several of them are supposed to be 24 or younger, further limiting the pool.
• There is an internal market to trade them around, but teams can only have an average of 8 non-domestic players. Rules slightly vary for the cc US teams versus Canadian.
• The league is legally one business, and holds all contracts. The “owners” are investors in the league who have a contractual agreement to manage a team. It happens much less often than it used to, but you occasionally see things that appear to be league office meddling in player movement.