Excerpt from this article:
…Most US smartphone owners download zero apps in a typical month, according to comScore’s new mobile app report… One possible explanation is that people just don’t need that many apps, and the apps people already have are more than suitable for most functions. Almost all smartphone owners use apps, and a “staggering 42% of all app time spent on smartphones occurs on the individual’s single most used app,” comScore reports. New apps come and go, especially games, but perhaps breakthrough apps will be increasingly rare. A look at the top 25 most-used apps reflects mostly mature companies, including Facebook, Google, Pandora, and Yahoo.
Another likely reason is that it’s still not easy enough to find and download new apps.