I'm guessing its because of how it is emphasized.
The mantra drummed in to karate students is that the belt is the goal.
The mantra drummed in to BJJ students is that proving yourself through competition is the goal.
But competition isn't a requirement is it? When I was actively training BJJ, I really had no desire to compete. Actually, I had no desire to test either. LOL. I was there to learn.
I'm not into BJJ as others, so manybe some of the regulars can answer this question: How many BJJ mcdojos are there vs. Karate mcdojos? Seems like for the most part there is a pretty solid standard across the board with BJJ. I may be wrong in saying that, but its just an opinion.
IMHO, the goal should be on learning the art to the best of ones ability, not how many belts you can get in a short amount of time. Again, thats just my opinion.