[NotATroll]

I think it genuinely depends on what your users are nostalgic over.

You can look cutting edge while taking advantage of nostalgia.

To be entirely honest, the "flat design" doesn't look "cutting-edge". It looks basic, which is because, well. It is. Intentionally so. If users don't care what your buttons look like, why spend tons of time making them look a specific style. That's just more time spent.

I'd argue that the reason it's become so prevalent, is exactly that. Less time spent designing, more time spent on features.