Why doesn't GA aircraft use the JASC/ATA document standard?
Newer GA aircraft do generally follow this format like the SR22 service manual. As jamesqf mentioned in the comments many of the popular GA aircraft were designed upwards of 50 years ago and their airworthiness and maintenance procedures have not really changed a whole lot thus the manuals are sill what they were when published 50 years ago. 

14 CFR § 23.1529 requires that instructions be drafted (or in the process of being drafted) during certification. Like anything in aviation once its certified, why rock the boat by re-writing the manual just to comply with a format. 
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