With all of the layoffs and downsizing and economic struggle we've been having, a lot of people have lost their jobs. As a reduced force, people who are left are expected to be able to do the jobs of 3 or 4 people at once and be happy about it and grateful that they at least have a job.
When the companies have started hiring again, they are (erroneously) assuming they can find candidates who now have all of these very specialized and different skills, in one person. . . a Purple Squirrel.
Prior to the layoff, Acme Company had 1 Project Manager, 1 Computer Programmer and 1 Network Engineer. Due to the economy, the Project Manager and Network Engineer were laid off and the Computer Programmer was expected to learn how to do both of the other jobs and was successful in doing so. When the economy picked up, the Computer Programmer left for another job.
Acme Company needs to replace their employee and now they are looking for a candidate who has the skills of a Project Manager, Computer Programmer and Network Engineer... all in one. Unrealistic and unconnected, the client assumes that because they were able to create a person with this experience, there should be others out there. The quintessential Purple Squirrel.