I was looking for replacement batteries for my cordless tools (drill, light, sander) and have been unable to find any.
My current battery is the style with a neck that inserts into the tool, but all the newer tools have batteries without a neck that just slides onto the tool.
Why the change?
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.)
The neck style battery pack typically held an extra 1-2 NiCad cells in that neck portion. With the transition to higher voltages, they want to make at least some change in design so you can't use the wrong battery like @Henry Jackson stated. But the main reason you don't see any is the way lithium-ion packs are constructed, yes they do still have a few battery packs that are round and are fully inserted but the majority of lithium-ion cells are designed rectangular, wide, and flat. These stack better in a normal box form factor and the neck would not be used to store any extra cells. Most tool companies are offering the same tools (same voltage mainly) that work with either NiCad or Li-ion. The more rectangular the form factor takes the less space to make a mold and typically less plastic/material so without the advantage of using the neck to hold more cells, the inefficient neck form factor has bit the dust.
While I can't prove it, I always thought this change is because of the proliferation of many different types of battery power tools nowadays (drill, jigsaw, circular saw, light, impact drill, saber saw, vacuum cleaner etc.).
Back when battery power tools were basically just drills, the neck type batteries fitted them well because the handle would hold some extra battery cells. Other tools may not have well suitable handle for such a battery, while the cuboid format is pretty universal for attaching to tools.
Apart from that: if you can't get a suitable replacement battery, you can still:
For example, I used a broken spare battery pack for my Milwaukee 28 V tools to "add a cord" to them. This allows me to power them directly off 2×12 V car batteries while living off the grid, and for much longer than with mobile battery packs.
