Comment by Dagger2

They pretty much did. "Just add N bits to v4" is far more work than you're thinking it is, and most of what v6 does is a direct consequence of taking v4 and adding more bits to it.

The amount of work doesn't depend on the number of bits either, so adding fewer bits is a false economy. Deploying a new version of IP is so hard that you only want to be doing it once, not once every time you need an extra few bits.