Expectations have increased a lot in web development. When I first started working, web front ends for business applications were ugly. They were designed to do the job and nothing more. They might also be hard to use without a manual. These days the designs I get given look pretty and have simplified interfaces for the user. The designs never use native elements and often invent novel UX interactions.

Back when I started, the go to front end wasn't a web app either. It was some GUI on windows which involved pretty much the same level of complexity as we have today on current web apps. So maybe the question isn't really "why did front end development explode in complexity?" But really "why do we expect front end development to be simple when asking for complex UX?"