Why does this Happen

Players LP losses/gains are not supposed to be identical or constant. They are based upon the player's wins, history, division, and who they win and lose against.

Due to the amount of games and the relative movement of your account in its placement the algorithms has determined that you are sorta where you belong if not slightly higher than where you should be. This being said does not mean it is impossible to improve and climb it just means that it will be harder.

Your MMR is not high so the system thinks you deserve your current rank so it makes it harder for you to climb. Believe me I feel your pain.