SpaceX granted themselves the monopoly on these large scale constellations by being the first (and so far only) organization to develop (mostly from scratch!) reusable orbital rocket stages. Anyone else (without Falcon 9) launching the same number of launches/satellites would spend several large multiples of the amount SpaceX did, which makes it economically infeasible.

There will be other reusable orbital launch systems (although it looks like #2 will also probably be SpaceX, heh) but until that time everyone else is priced out of doing what SpaceX is doing.

It's got some analogs with TSMC and the latest process size microprocessors. Nobody else is prevented from doing it as well... it's just that they figured it out first and nobody else has developed equivalent technology yet.