Pure speculation here - i'm thinking they wanted to give Dave a Muppet Show character similar to his Wendell Porcupine from Emmet Otter and the idea ended up transforming into the dim witted janitor. It was clear by season two that George was being backburned - where he had been on every episode in season 1 (mainly thanks to At the Dance), the first season was largely an effort to throw all these characters at the wall and see what stuck - and it was pretty clear by the end of that season that Piggy and Fozzie were going to go on to star status...making it hard enough for the two to be in backstage scenes together - and as a janitor, George was mainly a "backstage character". He also was very similar in design (and somewhat in character and voice) to Statler. By season 3, i think the writers realized they could bring in a new janitor character without anyone really missing George.
By the way, look for a really ultracool visual confrontation between George and Beau in the opening pages of next week's Muppet Show comic debut!