They probably rewrote the entire taskbar because reasons and when the deadline neared, they cut features.
I would not rule out the possibility that the team that rewrote the taskbar does not include a single person who wrote the original taskbar and now nobody knows how to get drag and drop to work reliably with all formats.
I forget where I read it but I heard that this has been a big problem in Microsoft: it's politically easier for a team to get approval to write a whole new feature (to replace an existing one) than it is for an existing team to get approval to spend a significant amount of effort making big changes/improvements. I suppose it makes sense in a lot of ways to work like this but it can lead to situations where a new component doesn't do half the things that the old component used to do. I'd be interested to learn if that really is what happened here.
