5 man instances are overused because Blizz has been too lazy to provide as many dungeons as they used to (BC and Wrath had 16 dungeons each).
People can't fill up their 20 mans because a) the game is dying (yes, it is still quite popular, but dying none-the-less), and b) people are burned out from doing Normal, then Heroic, and then expected to do Mythic of the same old tiring raid instance. BC had 8 raids and Wrath had 9 raids. Say what you will about the quality of some, the fact of the matter is that you weren't running the same raid forever for the same gear you already had only a few numbers higher.
The actual solution is for Blizz to actually build content, but they are too cheap to do so.