You might as well ask, "How high is up?" Different companies have different reasons for their decisions -- some companies want to stay up-to-date and install the latest versions of software rapidly, some companies deliberately stay one (or more) releases behind the current version, and some companies deliberately stay with a particular version (either due to cost considerations or because of some feature being used that would be lost by upgrading).
If you asked the current versions of these software products, that is a question that makes sense. But the question you posed is meaningless since there are so many different answers. Every company does things differently and that's just the way things are.