There aren't any documented rules on BG promotions and I'm pretty sure that is by design. As a community we've unearthed some of them but as weaw pointed out there are other criteria that we still are unaware of. I've been playing this game for over four years and it has always been a game of discovery; giving you just enough info to go off of and experiment and figure things out for yourself. Because of that I'm still learning new things. Some people like myself love that style of game, others not so much.

There is a very good reason you aren't allowed to decide if you get promoted or not, it keeps managers from artificially keeping their glads in Primus longer than they belong. If a gladiator has reached the top spots in Primus it means they are ready to move on and start their next journey in the end game.