Although longer arms produce less tracking error, they also have higher effective mass.
In the 70s, there was a common believe that cart should require light tracking force. Most of the MMs in that era were tracking at less than 1.0g. Even MCs were tracking at less than 1.5g. In order to achieve that, cart must be very high compliance and they were poor match for high mass arms. Therefore, those 12 inch arms were slowly out of favor and disappeared from the market.
Nowadays, most of the carts are medium to low compliance. They can work well with 12 inch arms.
In the 70s, there was a common believe that cart should require light tracking force. Most of the MMs in that era were tracking at less than 1.0g. Even MCs were tracking at less than 1.5g. In order to achieve that, cart must be very high compliance and they were poor match for high mass arms. Therefore, those 12 inch arms were slowly out of favor and disappeared from the market.
Nowadays, most of the carts are medium to low compliance. They can work well with 12 inch arms.