Why doesn't GA aircraft use the JASC/ATA document standard? By international law large aircraft are required to categorize aircraft information using a standard called the ATA document numbering system (Air Transport Association).

The ATA document chapters allow a pilot or mechanic to find the same information for any airplane under the same chapter/sub-chapter number. Thus a chapter number for a Boeing 747, B737, or Airbus 380 will be the same. Examples of this include Oxygen (Chapter 35), Electrical Power (Chapter 24) and Doors (Chapter 52). 

A few examples of ATA chapter numbers...

ATA No. ATA Chapter name
ATA 00  GENERAL
ATA 01  MAINTENANCE POLICY
ATA 02  OPERATIONS
ATA 03  SUPPORT
ATA 04  AIRLINE USE Aircraft Handling
ATA 05  TIME LIMITS/MAINTENANCE CHECKS
ATA 06  DIMENSIONS AND AREAS
ATA 07  LIFTING AND SHORING
ATA 08  LEVELING AND WEIGHING
ATA 09  TOWING AND TAXIING
ATA 10  PARKING, MOORING, STORAGE AND RETURN TO SERVICE
ATA 11  PLACARDS AND MARKINGS
ATA 12  SERVICING
ATA 14  HARDWARE AND GENERAL TOOLS
ATA 18  VIBRATION AND NOISE ANALYSIS (HELICOPTER ONLY) 


About 20years ago the FAA strongly urged GA aircraft to follow the standard with the anticipation that it would soon become law - it never did.

Why has the International community not mandated 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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