1) Some front end needs and/or frameworks were developped by companies having a scale and pool of talent justifying the needs for new tools tailored to their use cases (extreme audience = need to address many user specificities + pool of talents means the best brains on this planet invent something: doesn't help in coming up with a trivial solution). React, Flutter, AMP.
2) During the zirp, coming up with some great tech was also a way to attract talents. Using such tech was (is) a way to be part of that group too. There is a trend effect. I am wondering whether complex front end tech stacks are justified when you can't hire the best talents and when you need to work faster to keep in business.
3) Micro service all the things leads to using API's everywhere which leads to have consumers everywhere including your front end. Coming back from this could mean using simpler apps and dropping some of the front end complexity.
Probably more :)