Theory: the new store will be a unified global store rather than the patchwork of regional stores we have currently. This would go some way to explain some of the changes. PS3 and Vita content libraries are too different across the regions to integrate nicely today so they will keep the old stores up for those consoles but not the web versions.
PS4 content has always been more closely aligned between regions, apart from avatars and themes which had different policies in different regions, this is why they're not being carried forward to the new store (because it's not worth the cost of integrating).
There will probably be a new wishlist feature on the new store which also appears on the PS5 as well (not just the web), but they're not transferring old wishlists across from the different regions.
The great thing about a global store is it will enable them to implement features such as gifting, which would have been too messy to do with the old regional system.