There are millions upon millions of computers running Windows XP in circulation. Pretty much everyone I've spoken to, from laymen to reps of financial software companies to people geekier than myself, considers Vista a dud or at best a waste of resources. The conversion stats are up for question, but W3 estimate about two-thirds of machines they see are running XP —
Although most hardware sellers out there ship Vista on desktops, if you buy a small laptop (i.e. a netbook) right now and don't go the Linux route (most people want applications they recognise, so despite the huge progress Linux has made a lot of netbooks have been returned simply for not having Windows) then you'll have XP. Ergo it's a current operating system, and people who buy it now expect to get a good few years of use out of it. We're also in a global recession, so people want low-cost computers; Microsoft is currently talking about an OEM end date for XP Home on basic machines as either June 2010 or one year after the general release of Windows 7.
More philosophically, XP isn't just not regarded as old, in terms of branding it's ageless. Windows 95, 98 and Me (and 2000 and 2003 on the business side) have "obviously" dated — but XP isn't a number. No surprises that Office got off XP branding as soon as possible and back to 2003/2007/etc.
We've entered an era in which updates of software and operating systems rarely add significant new features, and most people are aware they're being shafted by forced obsolescence on the part of Microsoft and computer manufacturers. Businesses have only just about standardised on XP by now; we're still using IE6 at work, like millions of other locations internationally.
The problem comes when Microsoft tries to end development of patches and updates for XP. They're going to get crucified if there's a sufficiently large number of machines active that get successfully targeted by organised online crime as a result, and that's not even taking into account implications for economies the world over and national security issues.
Currently that date's set at April 2014 —
— which gives Microsoft less than five years to get Windows 7 deployed very widely indeed; it's currently passing off responsibility for XP support to machine sellers (on the basis five-year mainstream support has already ended and we're in the five-year extended phase) so no-one will be surprised when those sellers lobby heavily for security patches to continue to be released after the intended end-of-life. Owners of machines bought new with XP now are going to be extremely pissed if their investment becomes an (even more) unsafe user experience in four-and-a-bit years.
The fact it seems to install and be usable on a lot of machines (with a gig of RAM or so) currently running XP will help. Okay, most will be pirated copies rather than retail upgrades, but it gives a route to make older hardware useful for those that won't switch to another OS altogether.