They simply want to make it harder for players to win. The probability of winning a parlay in the same event is higher than different events. You can take basketball for example, you'll have less problem on moneyline bets placed on GSW right now because that would most likely win. You just need to add one or two more to increase potential win (sportsbooks doesn't want that).
Is it really easy for players to win with a parlay in the same event? I'm still confused. The probability of winning is always about the odds. It is not about whether the bets parlayed are within the same event or not. After all the bets are independent of each other like the over/under and the moneyline bet. Which team wins doesn't have anything to do with whether the winning or losing team will score above or under certain points.
If I stick to winning probability in my parlay bet, I could still do that even if I'm restricted to different events. I could parlay bets with odds as low as 1.05 or 1.10.
If I'm already planning to bet on a parlay from the start, it would be good if I can already eliminate one event. Since GSW is dominant in this playoff, it's easier to take them on moneyline than their opponent. I can later add to the parlay the total score (over/under) which I find harder to predict and some other special lines for that game. This parlay also becomes easier because I am only considering two teams.
With the current rules by sportsbooks, you'll have to add more teams to the equation and that can increase the probablity of losing the whole bet.
