I explained this already:

-People could send from exchanges, and then they cant be refunded
-If the indexing function is not properly coded then it wont work
-If the players change their addresses in the meantime, it wont work
-If the code gets stuck in the iteration function due to some bug from the miners, then it can break, ethereum is still alpha and this could happen, its better to run it only once.

For example if a dividend gets stuck in a for loop thats a small deal, but if the entire balance gets stuck, thats a big problem.