For the same reason that Ra's Al Ghul's regeneration and resurrection abilities have been reduced to trickery, allusions and a mere philosophical immortality of his presence and why various plot elements or characters were changed from the comic source material. To bring him more in line with the movie's style, atmosphere and setting. Nolan's take on Batman is centered around a very realistic and serious imagining of the whole Batman universe, like all this could actually exist easily in our own reality.
And you certainly hit it on the head when you say "the Bane in comics was hilarious". Especially the whole Venom aspect is way too over-the-top for this. While the movies might really scratch the edge of technological plausibility at times (stupid cell-phone radars anyone?) they are far from the domain of Science Fiction or Fantasy, not only regarding the tech, but the whole style and presentation as very serious crime thriller dramas. Sure, the Venom aspect is not too fictitious, but the whole depiction of a half-naked muscle machine driven by huge tubes pumping his body full with some magical super-soldier serum is just a bit too over-the-top to fit into the serious and realistic universe we've been presented so far.
He also doesn't seem to see much reason in putting on some strange Mexican wrestler mask for disguise or threatening. Add to this also the not to underestimate fact that in a movie seeing the actor's eyes gives him much more ways for expression and can add way more to his presence than a weird mask. He also seems to prefer more useful and protective clothing.
But still Bane wasn't changed completely:
- In his answer Chad already alludes to the fact that even the comic Bane was quite intelligent.
- His background as a man grown-up in prison is still there (even if his geographic origin was altered).
- The Venom aspect also was not entirely dropped but just heavily toned down into a mere face-mask supplying him with anaesthetics or painkillers. And you could actually argue that those painkillers/anaesthetics are indeed giving him unnormal strength and endurance in the way that increased tolerance to pain enables you to go far beyond your normal capabilities. So it is pretty much a realistic interpretation of Venom. The mask even slightly resembles the original a bit.
- And also the movie-Bane is still quite a heavy chunk of a man who easily trumps Batman in a physical fight and who doesn't hesitate to show us his muscles.
He was simply adapted for the movie's setting. And we wouldn't want to see this in a Nolan-movie ;-):