I think fashion has a lot to do with it..................when I were t'lad traditional wets made up the vast majority of the river fly box and they caught fish no prob's.
New materials, access to new fancy patterns from all corners of the globe, tales of new nymphing methods (for example) had us searching for better, more innovative, holy grail flies and techniques.
Truth is the insects haven't changed and the fish know it - traditional spiders and wets are very much in vogue