The worst thing about being really good at something is being really good at it. Consider a DEPSCART expert. Years ago, when DEPSCART (ok, I made that one up) was common, lots of people could do it to a basic level. Over time, some people became much better at DEPSCART, and others switched to the next big thing, for example, AutoMiracle for Windows (AM4W - also fictional). Expert DEPSCART programmers can now charge extra, and AM4W programmers are competing against cheap graduates with the same experience.
Recently, more people switch to the next big thing, e.g. Mantra, and those still with DEPSCART are getting really quite good at DEPSCART; expert even. Really expensive. The DEPSCART-to-AM4W people switch to Mantra, competing with the graduates.
The DEPSCART-to-AM4W-to-Mantra people are good at all three, but not expert. They may be better programmers, as they have more ways of looking at things, but they are not as expert as someone staying with DEPSCART. DEPSCART consultants are rich and in charge.
The problem is, companies are not using DEPSCART so much anymore. There are no big exciting projects being started in DEPSCART, so work is maintenance based. Projects to rewrite legacy DEPSCART programs in Mantra are being created. Suddenly DEPSCART isn't needed so much, and the experts have to travel further to get less interesting, less lucrative work. Not having done AM4W, the jump to Mantra seems enormous.
The sell-outs who hop on each bandwagon are suddenly quite useful. "Yes, I used to do DEPSCART a few years ago, and I'm pretty handy at Mantra". Hired.
Clearly I'm generalising; there must be DEPSCART programmers who have got stuck in a rut, and don't get a whole lot better. Also, a DEPSCART expert is going to be bright (it's a tough language to master), so learning a new one won't be too taxing.
As with most things, the extremes are undesirable, and the compromise is usually best. The key here is to keep up-to-date, but not flit about too much, or get stuck with one thing. It's about being the most useful, not the expert.