The, (other?), major reason for the continued use of older browser versions is in major corporations where internal software solutions are written to be totally dependent on a certain browser version.
There were a lot of changes under the hood, (for good valid reasons), between IE6 & IE7, which would mean re-writing this specialist software for the "new" browser, plus the need to re-train users, all this on top of the actual upgrade process - not an easy thing to do when you are dealing with many thousands of users and systems
for these corporations it can take moths to confirm that it is OK to accept a simple security upgrade and to roll this out across the whole organisation.
If you frequent the technical downloads areas of Microsoft sites you will often see optional group policy downloads to prevent the automatic downloads to user systems, of compulsory downloads that can be applied by the IT divisions of these corporations, this shows how much this is a serious issue