It’s this, however also, firefighters are statistically the most likely people to be arsonists, and the lawyer definitely doesn’t want an expert on the jury who is also potentially sympathetic to the defendant!

Yes, generally speaking, the more intelligent you appear the riskier the trial becomes if you’re judging. If they want experts, they’ll pay someone they know will align with their side to show up. As for jurors, they want the average joe. Smart enough to put on his socks and drive to court, but also obedient enough to feel awe and respect for the institutions.