Why don't I taste the diet soda after taste anymore?!
Answers: After a while, what you considered an after taste becomes just the normal taste so you wont notice it. Also some manufacturers have worked to change the formulations so that this taste is less noticeable.
because you got use to the diet taste!
You probably consume so much that it doesn't even have that same affect at it did before. I use to drink diet pepsi all the time until eventually I stopped all together. After a few months of stopping I decided to try it again. I notice how it didn't taste the same anymore. It tasted to "diety" to me. I would have to start getting use to the taste again. Either way consider some flavored water instead.
formulas are changing or you are use to the taste
Most sodas are using Splenda sweetener now, instead of Equal of Sweet and Low, which had a very distinct aftertaste. The Splenda still has an aftertaste, but I find it's not as distinct as the other types of sweeteners that they used to use.