Most XP users use it because their current PC is good enough for what they do and they do not want to reinstall Windows or buy a new PC. If not for DX11-only games, I would still use XP (built a new PC in November) on my old PC. The 3GB RAM was a bit limiting, but not enough to 1) spend a lot of money on new hardware and 2) the pain of reinstalling Windows.
As for why Metro is bad while Android UI is good: Metro UI is good UI ... on a phone or tablet, but not on a desktop. Just like I would not use Android UI on my desktop, I will not use Metro UI too.
A tablet has a relatively small screen and is operated by touch. You need big buttons so that it is easier to touch them. A desktop has a large screen and is operated by keyboard/mouse. Metro UI places 5cm x 5cm or larger buttons, while I can easily click 1cm x 1cm icons, so it wastes screen space and makes me move the cursor further.
A tablet is usually used for one task at a time. I use my desktop with many windows open, most of them overlapping. If I had to use one full screen window at a time, I would be much much slower. I full-screen only two types of software - video players and games, everything else runs in windows that are usually considerably smaller than the screen.
The start menu takes up a small portion of the screen, but allows me to choose from many items. The start screen takes up the whole screen (there goes my context) and allows me to choose from a smaller list of items. Oh, and desktop programs are not on it by the way (at least for RTM Win8, don't know about Win8.1).
Another gripe just with Windows 8 UI - it gives no indication that some text can actually be clicked to do something.
Different interface for different devices (that have different uses). After all, I would not want to use this