In the early 90's Taco Bell changed the meat flavors it had since the 60's into a "newer" form. Taco Bell also used to have small gauge cheese, like at Del Taco, which changed to the current thicker grated cheese at the same time the meat changed. This change was undoubtably made to please the masses, but it results in an inferior product. Firstly, the smaller cheese melted faster and meant the beans didn't have to be so hot. The meat change was non-sensical. Around the same time, they changed to a new taco shell. Unfortunately, this taco shell used genetically modified corn which was later found to cause stomach pain in humans and the new shell was discontinued. I'm not sure if this was around the same time Yum was put together to spin off TB, KFC and Pizza Hut out of PepsiCo, but they were within a few years of one another.
There are currently efforts underway to petition Taco Bell to bring back the "classic" meat and cheese. The promotion, Taco Bell Throwback, would also bring the Pepsi throwback products into the restaurant. Pepsi Throwback products are beverages made with real sugar and not high-fructose corn syrup that has become commonplace in the last 20 years.