It doesn't have to do with picking faster characters..
Because as far as Heavy characters are concerned, I guarantee that most people will pick Bowser, Ganondorf, Ike, Dedede, even WARIO before Donkey Kong.
As far as character design and aesthetics are concerned, Donkey Kong is the plainest of the bunch by far and large. It's not an insult, I guarantee that most people like DK for what he is, but it's the sad truth that he is quite plain, even compared to his sidekick Diddy Kong.
Nowadays, flashiness = popularity. Just look at how popular Lucario, Meta-Knight and Wolf are becoming. Even ROB is gaining some steam after everyone got a look at his fighting style.
And it didn't help that DK's Final Smash is, some say, the worst aesthetically of the bunch(next to Peach in my book)
People take alot of pride in picking characters that not only appeal to their visual taste, but in some way personify them. I guess DK's just incompatible for most people when looking at the competition.. he is too humble.