Excuse me, but you are not showing any kind of evidence with those screenshots at all to support your words. However, I'm definately not saying you are wrong. There are several reasons behind it. Not only Warmane focused more regularly on wotlk and tbc, true, but also MoP was way less populated than the others in the era, I've seen an oficial graphic showing it.
Here:
But why we had full cata and mop servers then? Quite simple I guess, not only we have players who don't retail and still wanted to try them out, but also those servers were at their very beginning. As soon as they realize how the expansion itself works out, population starts to drop. And, unfortunately, the high amount of costs required to maintain the later expansions compared with tbc and wotlk is also a reason.