Can somebody answer this question?!?

Riot made some changes to placements in S5. Doing well in placements can't bump you up as much anymore from where you ended up after the MMR soft reset.

Almost everyone placed lower than they were at the end of S4.

What they said, but more specifically, everyone lost a lot of MMR with the soft-reset for the new season (150 MMR according to lolking). I took the liberty of checking your profile on Lolking and saw that your 3v3 Ranking last season was Silver 1...your 5v5 (the one you went 8-2 in) MMR was barely above Bronze. Your 3's and 5's MMR are separate you know?

I'm not completely sure WHY it happened that way this season, but I can tell you you're not the only one. One of my friends went from Plat 3 to Silver 1 and he went 6-4 with decent kdas. I think there was some article saying that they were changing the way placement mmr worked or something this season, and it ended up pretty wonky. So don't tilt too hard, it's not that you did badly, it's just that the system is a bit messed up.