This is actually a great question - one that I have pondered before. I will be curious to see if others comment, but my best guess is that a city has to be cultivated and it helps if there is an active Pro Domme scene already there. It's like the rich get richer.

Several years ago I traveled to Houston, which is the 4th largest city in the U.S. During my research I noticed there were very few backpage or Eros ads and only one substantial Domme (who doesn't advertise). I took this as a bad omen, but went anyway. I did okay but not great. The weird thing was that over half the subs I saw had never seen a Dominatrix before. The others had all served the local pro. Emails requesting a session or my return trickled in for weeks after my visit. It was like the subs there just weren't looking. Many found me too late.

That is an example of a town that probably could be cultivated, it's just too far away to make it practical for me.

On the other hand, I put out a few feelers about a Chicago trip (the 3rd largest city) and my email was blown up with requests. Chicago has a very active FemDom scene, several commercial dungeons and tons of visiting Dommes. And the rich get richer.