I think it's a matter of supply and demand.

Right now, more users have switched to doing things on their phones and tablets, so that is what developers are building. If enough people are willing to buy a desktop type app, then developers will sell that to them. But I think the mobile ecosystem offers a better range of possibilities -- you can charge for your app, you can go ad-supported, or you can give your app away (and have in-app purchases or extract value another way). With desktop, you have to charge or give your application away -- there is no ads or in-app stuff.