I’m not certain, but I believe it’s at the discretion of the moderators/site admin. If there’s a particular thread you think is “sticky” worthy, maybe try PMing one or a few of the moderators, telling them which thread and why you think it’s worthy and see what happens...can’t hurt anyway
In fact, maybe I’ll follow my own advice - there’s a thread I’m particularly found of that I’d love to see made sticky...guess I’ll try it too!