Microsoft has now confirmed that this is by design, and not just limited to the platform preview. "Windows Vista customers have a great browsing experience with IE9, but in building IE10 we are focused on continuing to drive the kind of innovation that only happens when you take advantage of the ongoing improvements in modern operating systems and modern hardware," a Microsoft spokesperson said in a statement.
Redmond dropped support of Windows XP with the release of IE9. Now the company is dropping support of Windows Vista with the release of IE10.
On the one hand, this is a good trend because it means Microsoft is not limited to older technologies when it works on the next version of its browser. It also helps push users to upgrade their browser version. On the other hand, the company is the first major browser vendor to drop support for Windows XP and Windows Vista. Furthermore, users who aren't upgrading Windows are less likely to upgrade their browsers anyway.
In short, most Windows XP and Windows Vista users will have to stick with versions of IE that are becoming more and more outdated. Alternatively, they will use browsers like Chrome or Firefox if they want to keep up with the ever evolving World Wide Web.