They don't want to report daily revenue because it would average out to a lot less than $900 million a year, I think. Two weeks ago they were making over 1 million a day, now it's over 2 million a day.[1] To me, that growth rate at this kind of scale is more meaningful than a yearly figure (or a daily figure, to be honest).

Of course, the reason they use "2.5 million per day" is to give you a classic Scrooge McDuck kind of visual, with a big semi trailer packed with money entering your palace every day.

[1] http://techcrunch.com/2013/03/28/supercell-2/