This is not meant to imply that there are no Intuitive engineers, or that Sensors can't be successful psychologists, only that they don't tend to gravitate to these types of occupations in nearly the same percentages.
From the essay itself (my italics). The author also says that often, Ns are interested in theoretical pursuits: and for me, although I am interested in science, it is entirely theoretical. I have neither patience nor motivation to carry out detailed experiments in keeping with scientific theory. I would much rather read about the experiments other people did. This takes me, at least, out of 'S' territory and into 'N' territory, as the essay defines it.
FP's explanation makes a lot of sense as well. Logically, if there is an equal amount of Ts and Fs in the population (it is estimated that there are), and there are more NFs than NTs (NFs are estimated at about 15-20%; NTs 5-10%), then there are logically more STs than SFs in the population, which backs his theory up.
It's a good article as far as the type preferences go. For making a rough estimate of your type, it would be very useful, I think. For actually explaining the theory behind the letters and the deeper theory of the cognitive processes, though...