Ego. An a warped sense of justice.
All too often they will refuse to help your lane because
1) They don't like your pick, so they want to teach you a lesson somehow by refusing to help you
2) They don't like that you got behind, and they are annoyed that you are asking for help therefore, to teach you a lesson, they will let you go down in flames for the rest of the laning phase.
The bottom like is, even in ranked, players would rather stick it to you, rather than do what it takes to try to win.
That, my friends, is what happens when unchecked ego's play this game.