Screen size is the biggest difference. Things don't, or at least don't have to, run full screen on a desktop. The UI is also not as big of a deal on a desktop.

Screen size is the biggest difference. Things don't, or at least don't have to, run full screen on a desktop. The UI is also not as big of a deal on a desktop. People don't buy Quicken simply because it is "prettier." Given the absurd number of different screen sizes, it's much more difficult to make an app that will look good on each of them. The second part is the API differences. You don't get to use the latest fancy new features when 34% of your audience is on the older version. That is similar to windows land, but it is also not nearly as big a deal as the screen sizes. Finally, hardware. Yes there is a lot of different hardware on desktops, but on average it is more powerful than on mobile devices. Ask anyone who has done embedded development, there are a different set of problems there than you have on the desktop. The same goes for mobile. Most of the new mobile devices have hardware on par with desktops from a few years ago, so it isn't quite as bad as embedded. That being said, the range of Android devices is huge, and there is some percentage running on crap that creates another headache for Android developers.