Why the sudden popularity of 12 inch arms

And, after upgrading to a 12" arm, the next step is linear tracking

True

On paper a 12" arm is superior, but when it comes to manufacturing one it is not so easy. I think the combination of technical advancements, lowered manufacturing cost (or conversely the reduced aversion to a high cost arm), and the need to expand the product line (as others have said) are driving this. We are a fickle bunch so the desire to have the latest and greatest has a lot to do with it to.

Size does matter

SME has been making 12" arm for years, Japanese especially loves the old SME 3012 and some 3012 were made with beautiful gold plating. Genesis168 pointed out the key point and that's why 3012 is a favorite of Japanese who value musicality over speed and detail.

SME moved away from 12" arm for years when they introduced SME V/IV/309 family, but now they are introducing 12" arm again. Market demand might be the reason for the re-introduction of 12" arm.

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.