why did i lose sp during a series for losing?

SP? im guessing you mean lp and if you have 100 lp and you get into a series and lose they arent just going to always let you get 1 win and get right back into a series lol

Cause you're still losing a game.

It's the same reason why if you lose your series you end up at 75 points or whatever.

I think if you go like 2-3 you'll end up 80 points, but if you had a bad W/L like 1-3 you'll be placed at lower LP.