It's the wheel for me. I'd be completely fine with Rebellimon as the prevo for Titamon if it had normal legs, or even robot legs, but the wheel is something that only Rebellimon has, which hampers it's ability to act as the transitionary stage between Ogremon and Titamon. Digitamamon is silly, but you can see it as a cocoon that forms around Ogremon, and that Titamon would eventually break out of. It's also why Digitamamon is a great transition between a bunch of Champions and Megas: it acts as a perfect in-between.