Why Can't I Get Silver

Seems like half my time in ranked is spent bouncing from 20-90 points in a division before getting to promos, and this doesn't bother me since it gives me more experience before I hit the next rank, what annoys the ever living **** out of me is when I play the best in my team, if not the game and my team makes the worst decision ever and throws the game while I sit there and tell them not to do it.
All I want to do is get out of bronze, is that so much to ask? the past 3 ranked games I've played were
Lux games where I won my lane then just farmed/pretty much won teamfights with my 4 man ults and all seems good, a perfect example of what ****s me off to no end is when, in one of the games I was 8-0 at one point but the enemy adc and top were pretty strong, we ended up raping teamfights but we had no wards, the support just built tanky leo with no wards and I was full build pretty early so I couldn't get wards, the rest of the team thought it a great idea to go get a baron with no ward…
All I want to do is get out of bronze, is that so much to ask? the past 3 ranked games I've played were
