I'm not sure what the requirements for delisting products are. It might be as simple as "this is old, and we don't want to be associated with it". Or as specific as "we've learned that it's not as safe and/or effective as it should be, and should be delisted for health and safety reasons."

But the real reason is almost always that they have a new version, which costs much more, and they don't want competition from inexpensive generics to their old product.

There have been some antitrust suits over that. I don't know how they turned out.