I have to wonder if it isn't due to the nature of the Android OS, it's 'free'. I grant not as many phone users are open source (read free) advocates as there are in the PC market, but I would guess there is some consideration of 'why should the applications cost me money, the OS is free' lifestyle. Most of the Ubuntu users I know wouldn't think of paying for applications. Surprisingly many will purchase a device with Windows on it and load Ubuntu...because it's free.
That said, I would guess developers make more off of iOS versions than they do off of Android, and certainlky WP7 versions, partly because of volume of platform, and maybe some because of user buying habits. You go where the money is. Interestingly I have read some blog entries by developers that indicate they are making more money off of their ad supported versions than off of their one time pay versions on WP7. Sort of makes sense, it's like royalties, the gift that keeps giving. Personnaly I'd rather pay for an app then get the free ad supported version, but the ones that have ads aren't terribly intrusive.
Does Android have an ad supported concept? How is that working out for developers?
Sometimes you are the anteater, sometimes you are the ant.