Because Wukong middle works. He has a strong early and mid game, which is strong in the current meta, good escape and mobility, also strong in the current meta, and has an amazing teamfight presence, also strong in the current meta. Combined with the popularity of running mana regen yellows or health yellows instead of armor yellows, combined with wukong's passive armor shred with his q, and you can get several easy kills during laning phase.
This argument of, "oh but he should go top", was the same argument used against Kha, J4, and Jayce mid. And look where that went. Heck, even mid Vi had a brief moment of use.
Mundo top is basically a split pusher, assuming he doesn't get hella bullied out of lane. If he doesn't use his full abilities, he'll get pushed out of lane. If he does use his abilities, he'll push up to turret and be prone to ganks. And every top laner and their mother uses ignite, so Mundo's ult is practically useless until teamfights, and by then there's prob a morellonomicon, or one of 2-3 ignites still up.