Why does match maker change for promo series?

I'm sure we're all sick of being lied to. Why does the quality of teammates drop dramatically for promotion series? This isn't idle speculation on my part. All the way up to series I'll be playing with mid level Platinum players against mid level Platinum players. When I get to series my allies become high Gold to low Plat against high Gold to low Plat.

This isn't the first time this has happened. Multiple times on multiple accounts both last season and this I've been able to track a notable drop in player quality as soon as I reach a series or a win streak ends.

If MMR is what really matters then don't force me to play with/against Gold 1 and 2's if for the last 20 games I've been playing with/against Platinum 2 and 3's; just because these games are more "important" than other games. The skill gap between a team of Plat 3 players and a team of Gold 1 players is cosmic. Games stop being about playing well as a team but who the hell is gonna make the first big fuck-up that leads to the snowball. I'm sick of having to just flip a coin to decide if I win or lose. I've played this game for over six years and I feel like I have less control over the outcome of any Summoner's Rift game than the 1% crit chance does of swinging bot lane.