Specialization tends to happen over time, and it's not just technology; it could be industry or type of company. It's probably more common for development areas than administrators.

I haven't really specialized in one area of SQL Server, but I have gotten more skilled, becoming a senior person. Which also limits the jobs out there. Lots of people would rather pay a junior/intermediate to run things if they can, and avoid the costs of a senior person.

I also have a friend who's a talented CCIE, but there are few jobs for someone of his caliber and most involve moving somewhere else. That's somewhat the price of success.