Why the sudden popularity of 12 inch arms

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

True

Well, I think, there are some reasons why the 12's are back.
Mainly based on marketing reasons, because they can be sold.
That's it.
The technical reasons, well, most manufacturers are not stupid, they know how to build a good arm, some offer both, sell both and honestly, who cares. Some of them say, no one needs a 12 Arm, some reviewers write, they can not live without one....
In a way, it is like Raul wrote, when you have the perfect match, no one cares. Some time ago I listened to such a match and honestly, I simply couldn't believe it. It was a old FR64 Arm with a Koetsu on a real super Turntable.
When I closed my eyes ( a "Blind test" ) I really thought " I'm listening to an Air Tracking Arm". But, the Table was really a good one.
Some Arms can compensate Errors from the Turntable/Cartridge more or less, but at the end of day I think, the 12s look cool.

The reason is, this Design can be sold. The world has to go round.
That's it.
And lots of Arms - even today- are not perfect, or have problems with bearings, work only well with some cartridges..., or the turntable is not able to show all differences or to bring out the best from every design ... there are various reasons.
When you don't believe it, buy a Phantom Arm and 2 identical Cartridges and the compare it. You will hear differences, but is the 12" (or all) really better?