Because you've reached a stable rating. Initially it changes a lot to become more accurate quickly, the more you play, the more accurate your rating is likely to be, so the swings aren't as big.
why am i getting low rating points for a win?
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Because of how often you play games.
Everybody has a special variable which determines how accuracte one's rating is. Over time, this variable becomes bigger and bigger. It just so happens that this variable determines the maximum number of points you get for a win.
The more often you play, the smaller your variable becomes (ie, the more accurate your rating is).
I get like mere 12 points for a win even if i play a player that is twice my rating or more. It makes my rating static and stuck at 1150 average.