When you have $$$ you can hire the best.But in OJ's trial, there were competing experts, with one testifying that some issues in handling the evidence effectively "contaminated" things so the results wouldn't be reliable. The jury that was seated could not really assess the competing experts because their schooling did not give them the necessary background knowledge. Now that DNA evidence is more common (and now that many schools address the issue in all science classes, even those not for college prep people) jurors are better equipped to deal with it.
It’s been a very, very, very long time though since that trial though.