They don't have satellite-to-satellite links yet, so to serve a customer the satellite also has to be able to see a downlink base station below it at the same t

They don't have satellite-to-satellite links yet, so to serve a customer the satellite also has to be able to see a downlink base station below it at the same time, and there's only a handful of those in one geographic region. If they doubled the number of base stations in a new area (with satellite coverage) they would double the number of customers they could serve right now. So the constraint at the moment is mainly the number of base stations. The thing is once they have the sat-to-sat laser links working, they will be able to scale up with a lot fewer base stations, so there's no point deploying masses of base stations right now.