Ding ding! This got buried a few pages back, but that part I bolded is the biggest reason for making the change. For twenty some years, we've had the Great Wheel cosmology. And, it's never been overwhelmingly popular. Sure, it has its followers and it has some great writers, but, it's never really made it onto the average gaming table.
This, IMO, is an attempt to make the planes a viable place to adventure for EVERY group. Suddenly, I don't need decades of out of print books to play a canon game. I don't need to go hunt through the used bookstores to find the lore of Yugoloths.
I can use the planes, right out of core without worrying that some long time player is going to stand up and tell me I'm doing it wrong.
Think of how this gels with the 3 tier leveling system. The assumption is that third tier (20th -30th) is going to go planar. Or at least have lots of planar sorts of issues to deal with. If that's the default assumption of play, then you need to make the planes accessible to all DM's.