D&D 3.5 is a deeply flawed system, as is 4e, but they are both good systems. Someone may not like what they do, but they obviously do something well. Anyone who says either system is "bad" is flat out wrong. That's like saying Shakespears works are bad because you don't like Elizabethan English.
Remember that D&D 3/3.5 brought D&D into the mainstream and started an RPG revolution that created literally hundreds of new games, and dramatically improved the quality and variety of RPGs available, and 4e has created an environment where you can go to nearly any gamestore Wednesday night and play a game with people you've never seen before and have fun. That's downright amazing, and it didn't happen by accident.
That said, D&D does get a lot of players because of it's reputation and marketing. I've seen a lot of people playing D&D who really should eb playing a different system for their game, but they stick with D&D either out of ignorance of other systems existence, or an unwillingness to learn a new system. D&D is like a pair of pliers, it works for many jobs, and some times it's even the best tool for the job, but sometimes a box wrench just does it better.