That's a very interesting point. How to do you attribute this modern narrowed set of candidates' backgrounds?
I'm not sure I understand your question. If you're asking how we came to this situation, it's the result of a variety of factors. I think it can be traced back to Reagan, who was the first president who intentionally chose nominees in their 40s and early 50s because he wanted them to serve as long as possible on the court. In that, he largely succeeded (one of his nominees just recently died). But in order to get those younger nominees confirmed, they needed to have some particularly glittering judicial bona fides
that would compensate for their relative lack of experience. That meant getting federal appellate judges who went to the best law schools. Not coincidentally, the Federalist Society had a bunch of guys who met those qualifications. And they were all
guys - when Reagan wanted to put a woman on the court, he had to reach down to the Arizona appellate court for Sandra Day O'Connor (I earlier said she served on the Arizona supreme court - that was incorrect).
So that set the template. Apart from O'Connor, Reagan's nominees (including Robert Bork and Douglas Ginsburg) more-or-less followed that template (Bork was 60 when he was nominated, but his nomination was payback for his role in the Saturday Night Massacre). Scalia was 50 when nominated; Ginsburg was 41; Kennedy was 51. Bush gave us more of the same with Souter and Thomas, and the pattern was set for Clinton, Bush II, and now Obama. The one nominee who broke the pattern - Harriet Miers - was so manifestly unfit for the job that nobody has deviated from the script since then.