Noting several recent appointees to the California Supreme Court who seem like potential future SCOTUS nomineees, Orin Kerr asks why we don't see this more often -- "this" being the use of a state supreme court as a farm system for the highest courts. The appeal of the route makes some sense -- it provides valuable judicial experience while not being hamstrung by Senate gridlock.
One good explanation, which Kerr floats, is that many state court positions are elected rather than appointed. I have two more:
(1) Not all state supreme courts are created equal. It seems to me that nominating a judge from the California Supreme Court would be viewed qualitatively differently than nominating someone from the North Dakota Supreme Court. The high courts of larger states probably would be seen as sufficiently prominent so as to render their justices credible nominees. It might not wholly be a function of size -- certain state high courts have outsized reputations for quality whereas other states ... lag ... but I don't think that every state high court would be considered suitable. Kerr gives the example of David Souter, who spent most of his judicial career in the New Hampshire state court system, but recall that he was first appointed to the First Circuit before being elevated to the Supreme Court. My understanding is that the former appointment was done because of a sense that it would be faintly ridiculous to pull someone from a tiny northeastern state and place him on the highest court in the land. And while his tenure on the First Circuit was brief, it does seem to be the case that even a quick stop on an appellate court is considered sufficient to wash away any "viability" doubts (see also: John Roberts).
(2) For the state court pipeline to work, a lot of pieces need to fall into place at once. Basically, you need a superstar candidate who lives in a viable state that has (at the very least) an ideologically-amenable governor who either personally harbors near-term presidential ambitions or is willing to carry water for someone else who does. The example of David Stras in Minnesota is a good one, but also demonstrates how the stars have to align -- if Stras had been on the University of Illinois faculty instead of the University of Minnesota, or T-Paw had not been elected governor of what remains a blue-leaning state, then that route is closed off. The federal system is much more flexible -- if one really wants to nominate someone, there will probably be a vacancy on the relevant circuit court or the D.C. Circuit, or a district court in a pinch, no matter what state they reside in. And there's no agency problem because the person looking to seed the field already is President.
Of course, it could be argued that (a) every governor imagines themselves to be President one day and (b) if Stras isn't around, they'd just pick someone else. But my sense is that only a few true superstars are thought of as SCOTUS candidates before they ever get into a relatively high-profile judicial role. It's probably not the case that Minnesota had infinite David Stras-calibur candidates for a future SCOTUS nomination waiting in the wings. Rather, I suspect that at any given point there are a handful of Stras types out there waiting in the wings, who may or may not reside in states that have the right combination of factors to make a state court appointment a viable waystation to the Supreme Court. Outside those few cases, our thoughts on viable SCOTUS personnel tend to focus on folks who already are sitting on a major court. Sometimes, like Stras or Goodwin Liu, they get lucky. But for the most part, the federal system just offers more opportunities and a tighter nexus to put someone in the Supreme Court conversation.