Just one person's humble opinion and nearly personal thoughts on the subject...

There's just one reason... "Regression Testing" and it's actually a whole lot more expensive than a lot of folks think just to get to that type of testing.

Microsoft has seen fit to not only deprecate and discontinue a lot of things but they also do things that don't make sense in a lot of cases. Because of that, you have to do some pretty massive testing to make sure things don't break. That, in turn, means that you have to bring a new server up because there's normally not enough room on one server to have two copies of the larger databases and you'd be a fool to try to do an "in-place" upgrade because "going back" isn't that easy and I've also heard of too many in-place-upgrade-horror stories. There's also not much time for downtime and that's another reason for not doing an in-place upgrade.

For example, remember the great migration from 2000 to 2005? First (IMHO), the RTM of 2005 wasn't worth a hoot and wasn't even worth looking at until after SP1. Then there are things like what they did with ORDER BY... that broke a lot of code. Yeah, I know what they said and I didn't get caught by it but I (and, apparently, a lot of others) think that an ORDER BY should be a set delimiter and it should bloody well work.

And remember all of the complaints that came about because they decided to change what the optimizer did n 2005? Code that used to work just fine and with good performance suddenly didn't work so well and required anything from minor tuning to a major rewrite. Similar problems existed when 2008 and 2008 R2 came out.

How about all the fun that DTS users had? Yeeee-haaa!!!

And, it's going to get a whole lot worse. Remember that there's still the "promise" that you won't be able to change settings concerning NULLs, etc.

When MS stops breaking code with their releases, that's when I think you might see an increase in upgrades. Maybe not even then because a lot of good folks correctly have the attitude of "If it ain't broke, don't fix it" or "With upgrades, you take the chance of jumping from fat to fire".