As someone who spent some time developing desktop applications, I can tell you that juggling between modern and classic can be a burden. When management wants something new and shiny all the workers will have to adapt, and that interrupts their workflow. It doesn't have to be a big difference, just changing the tab order on some form or hiding a function somewhere else in the menus will have them cursing. They tabbed through it blindly for perhaps a decade or more and now they have to stop and think for a moment, or they continue fucking up and see their career sagging from too many errors. Although sad, it's fascinating how some can work with computers for years and then get a nervous breakdown when they have to look through new menus and can't find what they need during a critical time.

From my perspective, main sources are new hotheaded devs who want to make the best new program, managers who equal their wants with the needs of their workers, and marketing - but good luck making any of them listen to a random programmer.

TL;DR: Programs aren't for users, they are for selling.