I have been playing Pantheon almost exclusively and as it is right now, I only get 2v2s. The last time I got a 3v3 or 4v4 was more than a week ago. My guess is they set it up this way to:
1.) Eliminate the chance of getting a 6 person game with one person not being able to join.
2.) Speed the connection process up (which is has tremendously)
3.) Stop the use of AI bots in larger matches. I haven't had a match with an AI player in over a week. Yay!
4.) Oh, and maybe b/c people are impatient waiting for 6 people rather than 4.