"but what about Google? it has lots of users that are not willing to pay for anything"
Alot of the reasons stated szymek are problematic for any developer, regardless of platform. There is a certain playing population that will never pay.
Perhaps we as developers are partly to blame. The current trend to make people pay to remove adverts as seen on alot of iPad games, is actually insulting to our customers. Basically we are saying, I have put an annoyance in your game, so that you can pay me to remove it. Perhaps we need to look at our ethics, and try engage our paying public on a slightly higher level. ie make a great game that people are willing to pay for.