My personal reason is that GTK 3+ lost his track in what constitutes a desktop widget set. It's also getting slower at each release, it keeps breaking visual consistency and basic features get removed or heavily deprecated in favor of a "dumbed down" approach. So if the non-technical users are actually migrating to dumb web-apps and couldn't care less, the technical crowd which demand power features and efficiency are getting fed up. As a developer, I certainly am. The trend started already with the latest 2.x series in GTK, not with GTK 3.
My standards for quality on the desktop are way higher than what the "web" is supposed to give you today. It must be fast, consistent, and don't have any glitch.
The looks are a secondary point (and GTK 2 engines, or Qt4 for the matter) already give you great customization possibilities, which are way beyond what most other systems can do already.
GTK 3 and QML (Qt5) would like to target the same space as web applications. It doesn't make sense. Applications which are written with a web UI in mind will translate very poorly on the desktop, and the opposite is also true. My experience with QML was abysmal, and definitely not worth the extra dependency. Maybe you think that you button must be big and fat and have extra space to be tapped on-to, but on a desktop this is all wasted space on the screen that I want to use for data.
GTK 3 is trying to innovate a bit on the UI front, but many of the changes come with usability regressions on the desktop which I don't find acceptable anymore. Overlay scrollbars are a good example. In principle, they're a great idea. However the implementation is riddled of bugs which makes using scrollbars with a mouse a pain.
The CSS theming is nice, but they keep breaking the styles at each minor release. It's also incredibly slow. Whereas GTK+ and QML aim for rendering in a GL context, the reality is that if you want predictable performance they're going to be useless anyway.
My standards for quality on the desktop are way higher than what the "web" is supposed to give you today. It must be fast, consistent, and don't have any glitch.
The looks are a secondary point (and GTK 2 engines, or Qt4 for the matter) already give you great customization possibilities, which are way beyond what most other systems can do already.
GTK 3 and QML (Qt5) would like to target the same space as web applications. It doesn't make sense. Applications which are written with a web UI in mind will translate very poorly on the desktop, and the opposite is also true. My experience with QML was abysmal, and definitely not worth the extra dependency. Maybe you think that you button must be big and fat and have extra space to be tapped on-to, but on a desktop this is all wasted space on the screen that I want to use for data.
GTK 3 is trying to innovate a bit on the UI front, but many of the changes come with usability regressions on the desktop which I don't find acceptable anymore. Overlay scrollbars are a good example. In principle, they're a great idea. However the implementation is riddled of bugs which makes using scrollbars with a mouse a pain.
The CSS theming is nice, but they keep breaking the styles at each minor release. It's also incredibly slow. Whereas GTK+ and QML aim for rendering in a GL context, the reality is that if you want predictable performance they're going to be useless anyway.