riot third party events

How does Riot decide which organizations are allowed to host third party events or not?

If Afreeca, Convergence, RedBull can host third party events, what is stopping ESL from hosting those kind of events? Does Riot not allow big organisations to host them because they are afraid that it could replace the franchising as the tier 1 events

Riot approves those 3rd party events and set them as official off season events. I'm sure ESL could apply to do the same