In my game, all meta powers come from another dimension.  Its called the Imagination, or the dreaming, or the metarealms...

A worm hole, or a ruby thread, connects the metahuman to the metarealm. These conduits have a "gravity" like effect on reality.  Meaning that other minds that in contact with metahumans can create a conduit to the metarealm a sliver of a bit easier due to this "gravity."

So the more these wormholes congregate in an area, the easier it is for others to form.  This is why large cities have large concentrations of metahumans.  And this is why the first one is hard to create, but next thing you know, there are swarms of them.