If the AR was supposed to have a piston, Stoner would have designed it as such. He did design a piston rifle the AR 18 which actually is a great little rifle.

To me, the piston is a hardware fix for a software problem. The biggest selling point is a cleaner bolt/carrier.
Face it, how many really enjoy cleaning a nasty bolt and carrier?

IMO a piston adds components that can fail/malfunction, mechanical complexity, weight forward on a barrel and cost.
Parts left off cost nothing, parts left off cannot fail.