GSC OU is a tier because it's a metagame we have always supported. Despite that not a lot of people play it, it's a Smogon metagame that is supported. I mean, it's even in SPL, an official Smogon tournament. Monotype is not the same, as it's just another metagame that some people like. Getting a metagame supported requires a lot more public (and staff) support than just a suggestion post in the PO forum. If you want to try to get something like that started, the place to begin is Dragonspiral Tower.