What made A bodies non-popular

The A's best year sales-wise was 1974 ; 1973 was a very-close second , again sales-wise .

Then , consider how inexpensive they were ; less-than $3,000 for a 340-powered Dart Swinger / Duster !
Prior to 1970 , one could order-up a high-performance , nicely-equipped Dart GTS or Barracuda Formula S ; this was the only way to get the 340 until the 1969 Swinger and 'Cuda models ( yes , the vaunted 'Cuda was a stripp-o model in 1969 ) .
Then the E-bodies roll around for 1970 ...
No more big blocks ; no more consoles with a manual trans ; no more FM radios ( until 1972 ) ; just plain ol' compact cars ... again .

I've read somewhere that the tail lights on the '71 Demon were originally intended for the 1971 Duster . This makes sense , as the tail lights' design would've flowed with the then-new 'Shark Tooth' grille's "teeth" .
Now there's a custom idea for you to go with ! A '71 / '72 Demon tail light panel and lamps on a '71 / '72 Duster !

Usually , when the dreaded corpoRATe bean counters *suggested* cutting-back on a project / option list , the A's got nailed first .
To wit :
Rallye Gauges : Dropped after 1971
8.75" diff : Dropped after 1972

Then , there's that stupid small bolt pattern for the wheels :
the A's didn't get the good , universal pattern until the high-perf models were gone ; 1973 was the first year ; same goes for the disc brakes : A's were stuck with that shitty 4-piston setup through 1972
( and then , 6 cylinder models were still stuck with both the small bolt circle AND 4-piston binders until 01/01/1976 ! ).

Many 340-powered A's were stripped of their motors , and tossed-out ; same went for non-Six Pack , non-Hemi E bodies and '71 B bodies .
Hard to believe , I know ...

There was a time when one could buy a running Dart / Duster / Demon / Dart Sport for $400 ; maybe $800 - $1,000 for a running 340 model .
Those deals are long gone !

The E-body programme was dropped mid-way through the '74 model year , as they were disappointing from the get-go . Victims of the 1971 recession , they never recovered sales-wise .
The Octobre 1973 Energy Crisis was the final axe in the E's roots , as A body production was ramped-up at Hamtramck ; the lines were freed-up for A-body production .