In his post on why a President or Governor might select a judicial nominee who has no prior judicial experience, Josh suggests that it may be because you can get better insights into the non-judge's views of the law based on the cases they have litigated as a lawyer:

One possible reason is that lawyers get to pick their cases. By contrast, lower court judges can only decide the cases that come before them, and in most cases, are constrained by stare decisis. Their opinions are not a reflection of their personal views, but--or at least should--follow the law. Advocates, especially those who engage in strategic litigation, are differently situated. They can advocate for their bona fide conception of the law, and are not subject to the same constraints. In many regards, non-judicial experience may be the most useful metric for how a judge will decide cases. This dynamic may not always be the case, especially for lawyers in government service. There was a time when John Roberts signed a brief calling for Roe to be overturned, and we al know how Dobbs turned out. But on balance, the cases a lawyer takes, and the arguments he advances, is a useful predictor of judicial behavior.

I am skeptical that this is the typical reason. Even people with prior judicial experience were non-judges once.  And if the cases a lawyer litigates are revealing, presumably the Presidents and Governors would want to pick among older candidates with more litigation experience who have really had the chance to pick their cases.  I gather that is not happening, though.

It seems to me that the more common reason Presidents and Governors pick someone with or without experience relates to the politics of confirmation.  It's mostly a question of math.  How many votes do you need to confirm your candidate, and how many do you have? If you need the votes of wary legislators or even legislators from the other party to get your nominees confirmed, picking nominees with prior judicial experience can give those legislators comfort and maximize your odds of getting the nominees through. You can point to how well respected the judges already are, and you can show the record they established.  On the other hand, if you walk in with all the votes you need already, you don't need to go with someone who is already a judge.  You can pick the person you want.