First, I can also only speculate, as I’m just another user. But it was already often discussed here and there on the forum - as far as I understand, the main reason was, that there was an “inconsistency” in the old extension, what clicking an item would result in.
If you clicked on any item shown in the “Tab” tab we had in the old extension it would auto-fill on the current site.
If you clicked on any item in the “Vault” tab we had in the old extension, it would either open the item view or open a folder, the types or the trash.
That confused many (or just some?) users, including me. To this day, I hesitate to click on an item in the new extension, because I’m still not sure what will happen. Slowly I get back the trust, that just the item opens every time.
A second reason may be the general change from two separate tabs (Tab and Vault) to one tab (Vault). Maybe the thought was even, that in one single tab this inconsistency would be more detrimental. I don’t really know…
Yes, the app stores have to approve it - and Firefox is always… not the first one to do that.