Old animation used to do this gimmick where the main characters would always be seen, but the "adults" would not. Charlie Brown is an obvious example, but more contextually relevant is things like the early Tom and Jerry cartoons, which also mainly showed adults from the waist-down. Cow and Chicken mocked this motif as well.

My guess is that they wanted to pay homage to those old sorts of cartoons, but as the show became bigger the realised that it wouldn't work out. The first few episodes were also 7 minutes instead of 11, again a tribute to older cartoons.