Why so few C3 4 speeds?

Not that it really applies to earlier C3's, but once the VIN's began showing what optional items the cars were factory equipped with, all the insurance company needed was the VIN to know all they needed when determining rates.  

I concur...  once they went to the longer vin it was all over with. age does matter of course..  I guess that is why I paid 305 per month insurance when I bought a brand new 98 vette right out of college? (23yrs old)

Bill, The low percentage of 4 speed cars is somewhat higher when you look at the book and add up the numbers.
I agree with what's been said concerning insurance, and with horsepower coming down the Corvette became more of a car of comfort than performance. Also, the automatic transmission was becoming a much better performing unit. The first pickup I bought new was a '74 Chev with an auto and I could not kill that thing.
For 1979 take the total production and deduct the automatics (MX1) and that gives you 12,353 4 speeds. With your L82 you got the close ratio (M21) which shows 4,062 of the 12,353.
Not the best highway cruisers with the 3.70, but they are a blast to drive.