I personally don't use the snapshots, as I like to wait for the full release before diving into the new content. However, I don't think their release cycle of these snapshots is shifting away from testing these features. It seems like they've made some decent progress on the development of 1.17 from what we've seen in the trailer, but to fit all of that into a single snapshot might be far too much to get proper testing from the community.
How I'm seeing it is a slow rollout of content that they feel is ready to be publicly tested. The new blocks would make sense there, as what they affect is far more contained than new world generation. Could be that there's too many bugs with the current implementations of the world gen, new mobs, etc. that by putting them into the public hands finding new bugs would be far and few between. Or, could be they want to do a slower rollout to fix the new features in stages.
I also feel like it's strange that they've started pushing out snapshots nearly half a year before the projected release date, so I can see how this is seen as a marketing plan. Though, if the feature is complete, doesn't hurt to get early testing than waiting to do all the testing at the end. I dunno, I feel like I'm rambling at this point, but just some food for thought. :p
