Because the Android Runtime (ART) is very necessary to run APKs, and APKs are the only non-iOS standard for packaging mobile apps that is supported enough to be

Because the Android Runtime (ART) is very necessary to run APKs, and APKs are the only non-iOS standard for packaging mobile apps that is supported enough to be viable. Without it, you might get some open source apps to run on a linux phone, but you won't have banking apps, clash of clans, or a million other things people really would like to have on their phones.