Well, I know on Xbox LIVE Indie Games, they have certain standards that they want you to follow. For example, they recommend that your game can run with a transfer speed of 8kb/s. This is because many people own Xboxes and not all of them have high-speed connections or even live in places where high-speed internet is available (what a nightmare! :-) ). So, in order to support as many players as possible, they have these, I guess you could call them quality of service guidelines.
Now, due to these limitations in bandwidth, obviously that limits the number of players that can connect. Especially since many games are host-based (i.e. one player hosts a game on their console and others join). PCs, on the other hand, are individually so different and unregulated, that it's really up to the player to take care of their own "quality of service".
There is another restriction on processing power, if a host console has to do all the work figuring out where people are, running the physics simulation, etc., it's possible that it can only handle a certain number of players before it starts adversely affecting the host's gameplay. This, of course, depends on the game.
On PC, often the server (especially for large 32x32 games) will be a dedicated server, meaning that no one is playing the game on the same PC running the server (hopefully), so the entire processing power of that computer (more or less) can be dedicated to running the game.
That said, big AAA titles on the consoles do have their own servers for high-player-count gaming and it's not impossible for a console game to support 64 players or even more. But they may still have the up/down speed restrictions for each player.
So I think, for the most part, it's wanting (or having) to provide a consistent feel for their games across as many countries and regions as possible.