Joe Pesci figures out from the photographs that Marisa Tomei took of the tire tracks that his defendants' car couldn't have made those tracks. But Marisa Tomei didn't realize it, even with her expertise in tire tracks. Why is this?
Lisa was not familiar with the details of the case when she took the photos.
When they were at the crime scene, Lisa was casually snapping pictures of random things--Vinny even points out later how useless most of the images are. At the time, she did not know what kind of car the defendants were driving or even that the skid marks were alleged to have been made by the getaway car. Had she looked closely at the tracks (and there was really no reason for her to do so) she would have known that it was made by a car with positraction, but that information wouldn't have allowed her to deduce anything about the case given the scope of the rest of her knowledge.
Later, after the facts came out in court, she knew the relevant details, but it had been a while since she took the photos and again, it's not clear she ever even looked closely at them to begin with. Vinny makes the realization looking at that one picture, but cannot testify about evidence himself since he is the lawyer and the judge would be unlikely to give him that much leeway, especially after his early shenanigans. He needs an expert witness and knows that, given all of the information, Lisa will make the right deduction as well, so he calls her to the stand to explain the evidence to the jury.