Phones need a much more limited set of apps than a desktop or laptop. Many(not all) people just use the phone functions plus Facebook, Maps, Twitter and a couple of other apps and other use cases are covered by web apps. Multiple studies have shown that most people use very few apps.
While with the PCs, if you don't have some applications(like say AAA games, Office, Autocad etc.), it cuts away more of the potential audience than on a phone.
Phones also have the need of "my friends are playing X, I want to play X too". Words with Friends and Draw Something being two fairly recent examples. Windows Phone doesn't have these, so when my friends want to play their social games with me, I have to use my Android tablet. Nokia has said the games will be released at some point with a two-month exclusivity deal for Lumia phones (which is ridiculous and hurts the ecosystem), but I don't think they've been released yet.
While with the PCs, if you don't have some applications(like say AAA games, Office, Autocad etc.), it cuts away more of the potential audience than on a phone.