Why have cordless tools changed the battery style?
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.