Because the game has an SP cap to prevent people from grinding SP to win.
Next update will have a weekly SP can (in place of the daily sp cap we have today) and once you hit the cap you will get 1000 SP instead of just 50
i hit the cap last night
Cap isn't that high. Should hit it in an average 2 hour game session
yeah but you know i usually only play 1 or 2 hours a day, its just yesterday I played dust from 10am until 1pm and hit hte cap then