Universities help I think. Stanford, Berkeley &c. Many great public universities and many great universities!
Seattle at one point had Microsoft as the hub, Amazon came on the scenes in 2000s - once they both reached critical mass, there was practically no stopping the growth because engineers had an alternative. Eventually we had FB/Twitter/Snap et al starting offices here and having massive presences which led to present day. If only the weather wasn't what it is, I think we would have reached this ~20 yrs ago.
I don't know if it is going to be Austin / Boulder - but once any of those cities build two equally good employment opportunities, it is going to be a real contender. This is what makes TSLA / Oracle move significant: till now what were outposts are becoming real hubs.