IMO a big reason why is that web apps don't have the idioms and affordances of desktop applications, so in the end, the ux feels wrong to users unless do you a boatload of extra work implementing and maintaining the multiple versions of the experiences your app provides. In many cases it was easier to just rewrite the whole thing in a desktop framework, where the desktop idioms and affordances are the default behavior, instead of trying to implement them in a cross platform framework. This problem only got worse when the OS updated its look and feel, your cross platform app wouldn't look new, like it would if you used the built in frameworks provided by the OS.

Then again, maybe I didn't read very closely at what this is announcing... This is just talking about a new better install experience, but I think what I said is still true. They are putting OS affordances in the installer, but when you get to the app, its different. Yuck.

Another thing that really sucked is when MSFT deprecated "click once" installers. Suddenly this "easy" way to make an installer became a huge headache. For some stable, legacy applications you can't even build them anymore without either installing an ancient IDE, or writing a new installer. When google deprecates these installers, these apps are going to be in a similar boat. Honestly I'd expect it to be even more painful, because the underlying web part of the web app is a moving target, too.