Why do i gain so little lp

I Have been on quite a hotstreak this season and im looking forward to the climb ahead. I had 82Lp , i win as Garen so i assume i will get into promos , but no i get 17 lp. My friends always tell me MMR is so important . So im wondering, Im silver 3 100 lp NOW , and i Have Gold 5 MMR why do i only gain 17.

(Everything i am talking about is from my past 2 games, i will not play anymore for the day jsut for reassurance of others. )