The problem is not that there is nothing to do, there is always plenty of things to do.

It's more of a combination of being bored of doing the same old content and knowing that said content will soon be irrelevant in some way and therefore the motivation to play decreases the closer you get to a patch.

People feel like they might as well just wait for the next patch to come out where things will be easier/better than continue to play with the rules we have now.