I did do some sales on the game, but yeah sales were low. That is not the only reason though. Although I think this game is fun for all ages, the people who would be most likely to buy it are kids and they do not generally have paypal accounts and credit cards. So this way everybody gets a chance to play, and the people who have the funds can make a donation if they wish, and those who don't have the funds, well they get a cool fun free game.
There is a little marketing to this too, as the game will be ported to iphone probably within a year, and the more people familiar with the game the better. It will probably sell for a couple of bucks for iphone.
But I really do want everyone to enjoy the game, I have been thinking about doing this for a while, just didn't get around to it. I have over 700 downloads of it on download.com, so its pretty popular, but most people downloading it are probably the kids without credit cards
So those are the main reasons why.