Why do you lose so much LP after a lost promo?

So because I got stuck with a inexperienced, feeding jungler in my promos, I deserve to lose almost 50 LP? I'm going to have to go on a freaking three game win streak just to get back to my promos. Then IF I make it, I'm going to have to win two more in a row to advance. I literally need to win 5 of my next 6 games now. That seems unduely harsh. Why do you lose so much for a lost promo? That realyl brutally punishes people for having one or two bad games in a row when they matter most. No wonder people rage in ranked all the time.