tmiw

United flew there from LAX with EMB120 turboprops (~30 passengers) until they got rid of them earlier this decade. Assuming that such were going out full (which may or may not be a good assumption), that'd still be less than 50% load if they had used E170s instead, or 25% with A220s.

Now, a major airline might be able to subsidize such a route (even if it were just one or two turns a day/week) if they were to ever try service. I doubt a new one would have such flexibility, however, especially since business travelers value stuff like frequency and being able to still get home in an IRROPS situation.

At least for me, SAN is 10 minutes closer driving than CRQ. Not to mention that there are more airlines going to more places from the former (even if Moxy or another airline were to try service at the latter). It's similar to how flying internationally from LAX is still more cost effective in a lot of cases (e.g. SAN-LHR in economy is 2x the price on BA's direct flight compared to LAX-LHR on one of several carriers).

Another recent example: I flew SAN-SMF and went to the Redding area by car, a 2 hour drive from SMF. I could have flown SAN-SFO-RDD on UA instead and avoided the drive, but that ticket was $200ish round trip compared to the $100ish AS charged for SAN-SMF (WN was similar, IIRC). And we needed a car up there anyway. Now, would UA lower prices if someone else also served that route? Sure, but is there enough traffic to justify it without killing service entirely?

LHR (as well as most of the rest of London) has mass transit that's far better than in a lot of the US. And its location also permits long-haul flights to most anywhere in the world. Not sure it's a great comparison.