WoW and mmos in general just don't encourage exploration very much. It doesn't contribute to making your avatar any better. So unless you're really into just having fun - which the game mostly does not encourage in lieu of running dungeons and getting gear and etc, a lot of these zones have just ALWAYS been abandoned, save for some event bringing people to them.
The reason the population is 15-20 in these zones as opposed to 60+ is because there are better ways to level nowadays, not that anyone cared to visit these zones in the first place and heirlooms ruined that.
That being said, I realize you're not blaming this entirely on heirlooms and exp boosts, I just wanted to clarify on that specific part of what you said.