Even before Wizards, TSR started driving D&D into the ground trying to scrape out a buck. They released a ton of box sets and new campaigns and new worlds, trying to sell more books. Gary left TSR around this time and didn't get a ton of money. Then Wizards bought them and did a good job of bringing a little energy back to the brand, but also began to release version upon version to sell more books, and added more and more complication and numbers crunching. Original D&D is a very inflexible system, you pick a class and roll your die and that's about it, you don't obsess over optimizing your character traits. The newer D&D is a Wizards points allocation system with all kinds of complexity that stat geeks can fiddle with.
Of course computers were the fatal blow for RPG's. I love computer RPG's, but they're really completed unrelated to what playing a real live RPG is like. It's sort of like playing solitaire vs. playing bridge.