Why do many full face bicycle helmets not conform to the ASTM F1952 standard? It seems to me that a lot of full face bicycle helmets do not conform to the requirements of the ASTM F1952 standard for downhill bicycle helmets. Some noteable examples from the best selling list of chainreactioncycles.com are the Bell Super 2R, 661 Comp, and the Urge Drift. All of these appear to be popular downhill helmets that do not conform to ASTM F1952. Why is it so that many full face helmets do not conform to this standard?
Short answer: they're not required to.

According to http://www.spkids.com/pages/astm-helmet-safety-standards-explained   this standard consists of these tests:


Dropped from two meters on flat anvil (iron block); drops from curbstone and hemispheric anvils placed higher than bicycle helmet testing
Three anvils mimic various mountain bike terrains for helmet drop landings
A lower test line than bicycle helmets in the back of the helmet ensures back and side of head coverage
Chin strap/buckle strength and retention tests similar to the CPSC standard
Chin bar is tested under ambient conditions


By comparison ASTM F1447 is "Bicycle and roller skating helmets"  and the tests are:


Eight sample helmets, two each in hot (up to 127 degrees), cold (as low as -1 degree), wet and ambient environments
Four samples (one in each of the conditioning environments) are impacted with anvils of three different shapes (flat, hemispheric and curbstone) mimicking biker terrain
Dropped from two meters, the peak acceleration of the force during impact is measured
The ambient helmet is subjected to the positional stability (roll-off) test
Helmets are subjected to the dynamic strength retention test (chinstrap and buckle strength)


ASTM is American, so assume its degrees F not C.

Summary, the downhill helmet has to pass harder tests than a bike helmet to be accredited to the higher standard.

However, that all costs money, which has to be passed onto the end user as higher cost on that model. 

To paraphrase "you can crash harder on this expensive helmet and have a better chance of survival than with a normal helmet"  so the thought process goes "I don't/rarely crash but I have to wear a helmet for law/competition rules/downhill park T&amp;C"  

If there were cheap helmets for sale that met no standards, but ticked the box for "wearing helmet" then they'd sell quite well.