The answer can be expressed in three letters: EPA. The only SL officially imported by MB into the US in 1979 was the 450, with the numbers given (actually I think the '79 was slightly better, but the numbers are correct for 1980). The 380SL in US trim, dating 1981-85, was indeed lower-powered than even the 1980 450SL. The 280SL's secret? It was made for the European market and wasn't limited by US emissions standards, and it did indeed have both more power and better mileage. Gray-market importers did bring in some 280SL's and add catalytic converters to meet emissions regs, but they much more commonly brought in the Euro 500SL.
US and US-legal autos in general suffered through the '70's and earlier '80's, until they developed the computerization and FI technology we started seeing in the late '80's and since.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)