I can see a few reasons for this.

1. Whenever Explorer.exe crashes, it takes down the desktop including the taskbar. They are probably trying to separate the taskbar from the desktop.

2. It's a new style and people expect to see a unique style with every Windows version change. Of course, if you really want to you can make Windows 11 look like Windows 98 with a few button presses afaik.

3 a) It potentially looks like they might start auto-hiding the taskbar by default which could be interesting. If they are and they allow applications to maximize to the full borders of your monitor, that could potentially be awesome.

3 b) auto-hiding the taskbar frees up real estate and if you put on a tin foil hat you can say that Microsoft is going to use that newfound real estate to show ads to users and will justify it because they only take up less space than you were missing before, it's no big deal, right? (This is highly unlikely and Windows as an OS hasn't really shown people ads yet. The most it's done is shipped with minor bloatware apps.)