There is a pretty obvious reason, nobody likes him. They just pretend to because he has been around since Melee. But realistically, no one would miss him. The only change I would want is his removal. But since that is about as unlikely as him actually developing a fan-base nearly two decades later, they could start by giving me unlimited bacon and a higher chance at a good hammer pull. The only two moves that anyone knows him for unless they are 3 and love his jab because it looks like how his fans react to his inclusion.
Oh, and Pac-Man is the most retro of all retro character. Unless you are not including successful characters upon the basis that they do not rely solo on their retro nature. In which case, congratulations, Game and Watch.
I take that back, I do like him, as my punching bag. Do not get me wrong, R.O.B. could replace him easily, but you can never go wrong with a spare.