Why the American muscle cars suffered downfalls specifically in 1971 and 1974?

I know that there were quite many factors in the early-70s that led to the downfall of the muscle car - shifting social attitudes, growing insurance rates, Clean Air Act, and oil crisis. But apparently, it seems that the muscle cars kind of had their "shelf lives" for those specific years.I would see 1971 as the "best before" year:For MY 1970, the powers were at top - Chevy's 454, then Pontiac, Olds and Buick each had their own 400s/455s; Ford had the (Super) Cobra Jet and Chrysler the 426 Hemi/440 Six-Pak.For MY 1971, some engines' power went down, but often not by too much (440 Six-Pak '70: 390 hp, '71: 385 hp), and some remained.For MY 1972, there were major changes: 426 Hemi was gone, and so was 440 from Challenger/Barracuda; Mustang also lost the 429; the horsepowers dropped below 300; Torino was redesigned into new "Gran Torino", and Chevelle became a "standard" car for MY 1973, etc.And 1974 would be the "expiration" year. (Although Mustang was already replaced with Mustang II for that year):MY 1974 was the last for Challenger/Barracuda (and also Charger/Road Runner as such), and for big-block Corvette.MY 1974 most powerful Camaro had 245 hp, but in MY 1975 it was 155 hp. With Firebird, it was 290 to 200 hp. via /r/cars http://ift.tt/2sVRZmE