A lot of the decline has to do with the desktop importing web-based UIs: documents with a hodge-podge of interactive widgets that glue them into a cohesive app. Every app can be different and can be optimized for its function. New desktop apps are often built with either web-tech or some new UI framework that tries to give developers and designers that same level of differentiation and task-specific optimization you get in a browser.
Honestly, it isn't surprising that web style UIs are so popular: they take a lot less time to build, and you spend more effort building functionality than integrating the standard actions your app is supposed to support (think the stuff in the File, Edit and Help menu). Also, most users do pretty well with webish apps. Better than they should and most of the time better than they do with a GUI app. As they say, Good Enough beats perfect almost perfectly.
Honestly, it isn't surprising that web style UIs are so popular: they take a lot less time to build, and you spend more effort building functionality than integrating the standard actions your app is supposed to support (think the stuff in the File, Edit and Help menu). Also, most users do pretty well with webish apps. Better than they should and most of the time better than they do with a GUI app. As they say, Good Enough beats perfect almost perfectly.