There have been (counting all the interim Secretaries of State who served for a day or a couple of months) over a hundred of them. If you reverse the question to "How many Secretaries of State served as President", you get your broader statistical sampling. Still a rump survey sample, I'll admit.
But I can "imagine" a causal relationship. "Fall Guy". The Secretary or State has to be the point man(woman) for the Executive Branch's international policies. It takes a real slick individual or a lot of luck to avoid getting tarred with the failures, because the White House is always going to take credit for the wins and throw people under the bus when the doo-doo hits the fan. When it suits their purpose, the occupants of the Oval Office gloss-up the achievements of the SoS.
The recent trend towards "electable" SoS might continue. Arguably, Colin Powell, Condi Rice, and Hillary Clinton are in that category. Powell's too old now, but many a Republican thought he'd be the ideal candidate in '08. Condi's still spoken of, 'though I doubt she'll run for anything, and Hillary's simply born to run (and is married to a guy who's just chomping at the bit to manage another run for the big prize).
But for the period from Buchanan (just prior to Lincoln's term in the White House) to 2000, there were largely unelectable and with very few exceptions (FDR and Reagan who stood by their man), the SoS has been changed out fairly often.