Speaking completely unofficially (I'm not responsible for the changes or any planning, I'm just the self-appointed messenger), I can come up with a few partial answers:
The structures of various components are changing, which requires some change to the CSS.
While new things are being implemented, it's useful to have them show up for lots of people to see what kind of acceptance they get.
Some features that weren't available before are thought to have broad enough appeal that it makes sense to make them the default. This is where input becomes valuable.
At some point, the number of themes would become unwieldy, at least to the point that it's hard to locate a good one, so the selection has to be limited.
That said, it may be that "classic" and "new" versions of some themes will be issued. I don't know; I expect it depends on feedback from the users.
Caution: Contents may have been coded under pressure.