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.