There have been a lot of changes in batteries for tools over the years. Different chemistries, voltages, capacities. Obviously the tool makers don't want you sticking a 20V lithium battery into an old 12V nickel-cadmium drill, so they make the shapes different.
Maybe some of the discount manufacturers try to turn over their battery types regularly to force you to by all-new, but the more professional lines like DeWalt and Makita tend to use the same battery for years. I assume after a while an old style doesn't sell well and then they discontinue it.
(It sounds like you may not have a choice, but if you need any more encouragement to switch to a new lithium battery system, consider that they charge very fast, provide full power right up until they die, and can survive many more charge cycles than older battery types.)