Because he's annoying, unlikable and even racist.
He's really no worse than 3PO or Chewbacca even, but the reason why they worked is because they made light or humor out of stressful situations, Jar Jar doesn't do that even though that was the attempt. You don't feel stressed in Phantom Menace because you don't care about the characters so there's nothing to be made light of and therefore, he comes off as an annoying piece of shit.
And again, the thing that pissed me off most was that the movie was completely racist.