Because you've won too much, get too much value on your bets and/or demonstrated that you are a long term winner. They're forcing you to be a more recreational bettor.
If the person wins too much then he deserves it, what would have happened if he bet on Real Madrid to win this weekend? He would have lost, there is no problem with customers losing their money but when they start to win all of a sudden it is "too much win so you gotta be more creative the way you bet so you can lose more frequently"? I think that should not be allowed, if at any moment I win enough to get to that level and asked to make a slip instead of individual, I will leave %100.