Because most traffic courts just have a judge, not a jury. The judge knows the defendant is guilty, so there is no reason to put on a show. When the jury is present, they have to carry out the 'presumption of innocence'. It the same reason that you can get away with leading questions and some hearsay in front of a judge. He'll just say that he can 'separate the wheat from the chaff' if an objection is made. In front of the jury, he would have to strike the improper comments and give instruction to the jury.
Yikes!