Other airports do have ads for tech companies (Seattle comes to mind) but the concentration is significantly lower.

Billboards are primarily focused on brand awareness since you can't update them frequently and you don't have amazing targeting/attribution data. They're also hyper-geographic specific. For tech companies that means that billboards are most useful when targeting either a specific conference or when targeting people that can make sales decisions.

Sf's billboard space is heavily used by the tech industry because A) you've got a high concentration of consumers for your tech product (developers, marketers, operations, product managers, etc) B) you've got a high concentration of decision-makers in terms of director/VP/execs/consultants going through that airport who may end up making a final decision on your sales. C) you've got a concentration of investors in terms of VC, private equity, angel investors. This helps bring in interest for that next round/acquisition. For your existing investors this means that they've got something to brag about. D) hometown pride. Companies tend to put billboards near where executives travel through since it's a reminder of the work that the marketer is doing. It also helps with hiring and media reputation.

If you compare SF to NYC or Boston, those other cities have a much smaller amount of their workforce in tech (22% compared to 11%). This is especially true if you think about the number of people transferring at these airports. The concentration of customers just isn't there, which is why NYC ads tend to be more consumer driven.