It is not hard. The app store usually take 30%. That is A LOT and that number is growing when you sell a lot of copies.
But you save money, because you do not have to print Discs (normally more then you expect to sell in case the game is successful), you do not need huge logistic to bring the discs to the customer... In the end digital seems to be a good deal for most devs...
If you sell a decent number of games dev costs should be lower if you do not know if the game is a smashing success and it should not sink the company if case of a fail.