It's mostly because Crowfdlower does some additional filtering: they have an algorithm that rates workers, and bases it's trust in the worker on how well he does compared to others.
It also does some magic on resubmitting a unit as long as the results aren't clear yet.<p>Also you can set some "gold" units: assignments you have done yourself, so you know the right answer. This can be used to test if the workers are actually trying, instead of just clicking random stuff.<p>While I could have built all that myself, Crowdflower already does this, and has a bit simpler API to boot, than Mturk itself.