Numbers: Android and iOS each have over a billion users, so it makes more sense to make dedicated apps for those platforms. Desktop platforms are much smaller, so it makes less sense to dedicate the resources.
Technology: Chromium is available on desktop platforms, which allows you to easily make cross-platform apps. Chromium is not available on iOS, because Apple has enforced a WebKit monopoly. There's not a great cross-platform app technology that covers iOS, Android, Windows, and Mac, otherwise developers would probably go for that.
Numbers: Android and iOS each have over a billion users, so it makes more sense to make dedicated apps for those platforms. Desktop platforms are much smaller, so it makes less sense to dedicate the resources.
Technology: Chromium is available on desktop platforms, which allows you to easily make cross-platform apps. Chromium is not available on iOS, because Apple has enforced a WebKit monopoly. There's not a great cross-platform app technology that covers iOS, Android, Windows, and Mac, otherwise developers would probably go for that.