S6 Preliminaries

I just went 7-3 in my prelims which I thought was pretty good. I played well and carried my team for four of the games and was the center piece for the others. I ended up being bronze 5. I know that your league is based on your mmr but I don't know what more I can do. Why even have prelims if winning doesn't really help? The lowest league I've ever been in is bronze 2 and finished in silver 4 at the end of the offseason. It's not too good but I thought I was improving. Now Im back to the very worst possible league. Is there anyway I can increase my mmr faster? Because at this point its a little frustrating.