Because a speed bump has a different purpose, and has some negatives associated with it (which is why you don't have speedbumps at every crosswalk). This should work even when people know about it, because the real point is to draw attention to the crosswalk. Probably the people who know about it, are less likely to hit someone in the crosswalk anyway, it's the people new to the area that are most likely to be dangerous.
Now if they put these everywhere, they'd stop losing their effectiveness.
