What makes this theater the premier location for Hollywood premieres?
The TCL Chinese Theatre, which is it's current name has "premieres" (some of which are for TV shows) about twice a month.
Recall however, that these will be the US premieres. Films are often on international release days if not weeks before a US release.
Reasons are varied but high on the list will be tradition and location
Hollywood loves it's traditions and the Chinese Theater has been the "go to" spot for the glitz of Hollywood showing off it's stuff for decades.
Location is even more important during Oscar season.
One of the qualifying criteria for Oscar eligibility is that it must run in LA for at least a week
A film can release all over the country, but if it doesn’t spend at least seven consecutive days in a commercial motion picture theater in Los Angeles County, it doesn’t make the cut.
There are other criteria which also involve being in LA as well.
Combine the two and the Chinese Theatre becomes an ideal choice.
It's got the location, the look and the history....and Hollywood is all about those.