Because end users couldn't care less if an app is build using native sdk or web technology; is it multi-platform or not. All they care is how it works. As for developers: some only know web tech and are not willing to learn native—their loss. Some buy an idea of developing universal web-tech based app for two or three platforms will save them time. I doubt that very much. Either you get a crappy app on all platforms, or you spend more time polishing it than it would take to go native for all platforms from the very beginning.