The actual reason is that switching to hardware-accelerated font compositing means that you lose out on subpixel antialiasing, because of complications related to blending. The classic way of doing subpixel text rendering means you need three distinct alpha channels, and most hardware only lets you supply just one. There are ways of hacking around this with dual-source blending, but such a feature was not supported at the time of Vista. In Windows 8 and up, DirectWrite can now support hardware-accelerated subpixel AA, and the OS does so by default.
But what was this crazy obsession with hardware-acceleration of every rendered character and its grandmother? While you're busy Flippin' 3D I get it, you can temporarily render aliased Wingdings if that makes your life easier, but at least for Pete's sake I just want to enjoy not tearing my eyes out the other 99.99999% of the time. If there was anything slow or poorly implemented in XP it honestly wasn't the taskbar font rendering.
(Not angry at you; just annoyed at the situation they caused.)
> DirectWrite can now support hardware-accelerated subpixel AA, and the OS does so by default.
No, it (still) doesn't. This is my taskbar: https://i.imgur.com/zzIusQD.png
Market context: Microsoft yearned to implement the Metro UI they'd been researching for a long time (Zune was its public appearance) — and Vista kind of soured people on Aero by osmosis; third-party Android applications always had consistency issues, so Google wanted to mandate something that would be generic enough to work (and be easily implemented) for all apps but also look distinctly specific to Android; Apple wanted to signal some shift to upscale high fashion (parallel to "everything is black glossy glass and aluminum" in its industrial design) and understood fashion-minded upscale as a mix of modernist typography with neon bright contemporary color schemes.
To make up for the boring visual aesthetic that often resulted, they recommended adding animations throughout apps. Most of these animations aren't very helpful, but they keep the user distracted for a moment.
What? ClearType was basically introduced in Vista and refined in Win 7.
Win 8 is when they start to remove it for various applications (UWP, and some others) due to its poor performance on retatable devices (tablets mainly).