Part of it is the language. Technical interviewers love hearing about how I wrote a cool program in Lisp, for example, but then never seem to want to hire me to actually write Lisp code. (I didn't learn Lisp just to show off.)
Part of it is the project. Virtually no software project I see these days looks interesting enough that I'd want to work on it. (Most software I don't even want to use, and even if it's free.)
There may well be other parts. But thinking back, I had one job where I picked the language and the project, and did have quite a bit of fun there, even though it was by far my lowest salary.
I would love to be paid for programs I want to write anyway, the way I want to write them. Still working out how to do that. :-)
Interviews are a drag, and none of them like hearing that it will take one month to become fully accustomed (in-depth knowledge) with their framework. They'd prefer someone with limited knowledge who knows their framework rather than one who's well versed in multiple frameworks/languages.
It seems to me that a better approach would be to put new hires into code maintenance first with a mentor who can help them get up to speed.....
But most of us know that HR departments are not always the best judges of talent.
Programmers who advertise themselves as "Ruby programmers" or "Python hackers" or what-have-you come off as inexperienced and one dimensional. The best hackers have used the right tool for the task at and, and the best hackers have solved a wide variety of problems to require different tools.
Finally I have committed what might sound like the ultimate sin and given in to the Java/Spring framework. I'm learning various aspects of Spring, should have a small project up on github and plan on "specializing" in this framework for good.
All my other fun work is going to be done on my time alone. I look at it as the 'get serious, time to separate work and play.'
The reason is there seems to be a large number of programmers who "want to be good programmers" that do this. All that it really results in is a shallow understanding in many things with an accompanying attitude that they're smart, well cultured, or have seen a lot of things.
Dabbling in many different things is killing your learning curve because of all the context switching and you don't ever get awesome at any one thing. Putting those technologies or frameworks on your resume and blog like Foursquare badges doesn't mean anything. What have you done? Then and only then do I care about what you know.