Interesting numbers, those. It definitely looks like Brutes are a little over-tuned* at the moment, but I believe a large part is to do with the huge numbers of spine/fire Brutes farming AE. The same thread shows over 6k Spines/ and almost 10k /Fire. Ignore those for a sec and you're still looking at... 9 to 10k "normal play" Brutes, maybe? Still much more played that the other ATs, no doubt, but not to such an absurd degree.
I reckon it's mostly that they sit bang in the middle of the sweet spot between tankiness and damage and also in a really nice sweet spot for single-target vs AoE damage. So they kinda push the other melee types out simply by existing. Of course, the Fury mechanic also plays a big part in their popularity. It's easy to use in an optimal fashion (unlike Stalker crit), reliable (unlike Scrapper crit), and makes BIG NUMBERS (unlike Bruising). Especially in the early game, Brutes throw down some really big numbers compared to other melees and while this evens out as you get closer to 50 and slot powers up, initial impressions count for a lot.
Now, to what degree this is a legit problem for the game and playerbase? Is it something that the devs should be looking into and thinking about how to correct? I honestly couldn't say.
*Not necessarily in the strict numerical sense, but almost definitely in the player base's perception.