Primarily because you can't upgrade direct from XP, but also because it transfers lots of things that you accumulate over the years on an old system which are no longer needed. Sometimes settings and incompatible applications are also moved across and it makes for a generally slower system which is much more likely to have strange compatibility bugs. Some people that have done forced upgrades from beta builds to the RC have had similar problems too.
It's still perfectly possible to go from Vista > W7 and plenty of people will do it without a problem, but if you can spare the extra time then it's better to do a clean install IMO