Because the rise of streaming directly competes with long form let's plays? If you want to talk about gaming related content its either streaming, let's plays or commentary.
Also let's plays "evolved" even more into these heavily edited 30 minute videos out of 2-6h footage and most channels can't afford that. Look at channels like Real Civil Engineer and say how are you going to make a "normal" video about CS or whatever while competing with a dipshit that edits his videos for 12yo children? You can't afford this much editing unless you're full time.