The primary bottleneck among most top-tier VCs is partner time and travelling is a huge time-sink. Hence why most VCs tend to focus over a relatively small areas.

If a VC is looking at investing in a startup in a location where they have no activity then they're going to have to do trips out just for that specific startup (for due diligence, board meetings, etc.) and they're also going to have a harder time providing value-add (lack of local contacts, etc.)

Hence for a startup outside of a cluster to be worthwhile it has to be significantly better than the local dealflow and the deal has to be of a significant size. So such deals are always likely to be a rarity.

(Having a founder who's already built and sold a billion dollar company also helps immensely; if you're in that small category of people you can probably raise money regardless of where you're based)