Well, there are few Pokemon who can really make good use of Belly Drum.
One of these Pokemon is Linoone who had the ability to really wreck some teams in 5th generation NU. Linoone has pretty frail defense as well at 78/61/61. However, if Linoone gets the +6 then it is known for cleaning up teams with a powerful STAB Extremespeed.
Another one of these Pokemon is Azumarill. Azumarill just recently received Belly Drum which is great with its ability Huge Power. Now after Azumarill gets the +6 from Belly Drum it is know for ramming everything with a STAB Aqua Jet.
But Marowak doesn't get a STAB priority move, in fact, it doesn't get any attacking priority moves! This is not good especially because after a Belly Drum its slow base 45 speed coupled with 60/110/80 defenses will not keep it alive to do much damage.
TL;DR: Marowak is not an effective user of Belly Drum like some other Pokemon.