Why the sudden popularity of 12 inch arms

Zieman, it's not my place to walk Raul's dog for him so I'll offer a comment for all to consider.

You may have changed only one component but you changed multiple parameters. Not only degrees of tracing error but also mass, stiffness, bearing design and quality, resonant frequency, and wire (considerations just off the top of my head). I hope no one would disagree that you heard improved sound. But so many factors are involved that it is not fair to attribute that improvement solely to a longer arm.

From reading Raul's post slowly, that is MY conclusion.

Zieman, I have no problem with your answer to the OP. The point I was attempting to make was the caution that a 12" arm will not automatically be an improvement compared to a shorter arm. Some might infer that from your post, particularly with your emphasis that only the arm was changed.

There are many design/construction differences between your 312 and your Lustre and I believe that was one of your points. That supports the position Raul has stated many times - many elements come into play in matching a cartridge with different arms. For whatever combination of reasons, your cartridge sounds better in your 312. One should not assume it was just because your SME is 12".

I believe you understand this so I was trying to offer a clarification to readers with perhaps less experience. Peace.

"light weight compared with drums, was the main advantage." What about resistance to fade and ability to work in the wet?

Now we have REALLY gotten off topic, so back to the OP. My guess is that some manufacturers (starting with VPI I believe) saw an increasing availability of tangental arms as new product development responded to the resurgent interest in vinyl. Since the biggest advantage with tangental arms is reduced tracing distortion, decisions were made to develop arms longer than the standard 9" that might reduce such distortions without involving other issues for tangental arms. The fact that new design, material development, and manufacturing processes could overcome some of the high mass and rigidity issues from longer arms manufactured 30 years ago has been a bonus.

A second factor could be the overall upgrading in resolution of tables and cartridges so that performance of tonearms has been demanded and appreciated, whether by 12" or tangental arms.

And peace Raul. I understand that in your experience, you have not found such product development to outweigh basic system matching/compatibility. But that is another story and does not address the OP question.

Downunder, looks like you question as to why new 12" arms has a few plausible answers.

But it seems like the question of "better" could only be answered by direct comparison between 12 and 9" versions of the same design for (current) VPI, SME, Kuzma, etc. Even then, I'm not convinced you would find an ABSOLUTE answer. After all the selected cartridge matching with arm mass variance would still come into the equation. So I'll suggest there is no overall answer to the second part of your OP, but with a given cartridge one of the new 12" arms could provide the best performance.