The basic rationale dating back to the 70's is, simply put, that ATC personnel handle a constant flow of many planes, while pilots only manage one plane.
From "Review of the Scientific Basis for the Mandatory Separation of an Air Traffic Control Specialist at Age 56", page 2 (10th actual page) (link to pdf)
At this point, Mr. Francke identified several reports “proving” that the business of air traffic control was highly stressful. He first contrasted the description of the pilot occupation as “hours of boredom intermixed with moments of sheer terror” to a description of the ATCS occupation:
The controller has no such hours of boredom; he must frequently work for 8 hours at a stretch without rest periods and gulping his lunch at his position. Because of shortages of personnel, many of them have been on 6-day weeks for several years. This situation is further worsened by the fact that air traffic control is a 3 shift per day, 7-day a week operation, and each controller must take his turn standing midwatches. This is usually not too onerous on the younger man, but older men usually have difficulty in changing the rhythm of their life, and in sleeping during the daytime in the periods that they are on midwatches (Hearings, p. 98).
This will comparison may be outdated and a little exaggerated, and depends greatly on where the controller is stationed or what kind of routes the pilot flies. But one might very well argue, that an aircraft is a more stable system to handle than, say, a CTA is.