More simply put, stability is not valued in the front-end world.

It's interesting to compare the dominance of Microsoft in the enterprise world where IT runs the show was established largely due to stability. IT is run by managers who can get fired if instability costs the company money.

In the front-end world managers can't keep up with the changing landscape and therefore cannot make stability based decisions (in terms of software stability). Instead, in my experience, they make decisions based on how easy it is to hire (as cheap as possible) developers.

So, as a proxy, popularity drives the front-end because developers who have the time to keep up with the latest trends (IE mostly young 20-somethings) because they're the ones who get hired.