Because mobile phones' hardware and software are much closer integrated and closed systems than the desktop or the laptop. Bringing the web (or web tech) to phones is both technically more of a challenge and economically in particular Apple opposes it. Ceding control to the open web couldn't really be resisted on desktops.

However I'm relatively confident that the web is going to eat mobile regardless, starting with web technology and cross platform JS based frameworks gaining popularity, regulatory action against platforms, and when we're at the point where the web on mobile isn't disadvantaged any more it should be the clear preference.