One minor reason could be lack of physical media. In old days people received a bunch of disks or a dvd with nice cover art which they can put on a bookshelf.
Now it is all 1's and 0's coming over the wire(less). There is no sense of ownership. People are reluctant to pay for something that they don't touch or own.
One reason could be that consumers aren't technically owning anything, but merely being sold a license to use an application/movie/music for some random amount of time.
Here in the EU, the CJEU rejected the 'it's only a licence' argument and characterised software downloads (that aren't explicitly limited to a fixed length of time) as normal sales, in Oracle v Usedsoft -- in particular, they're sales for the purposes of the First Sale Doctrine. So I could buy your app, use it, then extract the apk and sell it on to someone else, and you couldn't sue me for copyright infringement (as long as I remove the apk from my own phone, of course). (IANAL).
I'm not familiar with US law but my impression is that, on the whole, most states' legal systems are a whole lot less consumer-friendly than the EU, so presumably the same is not true over there.
And in the Appstores case the free alternative is even legal!