For hunting, carry whatever is comfortable and handy in the field, where I think barrel length is the least critical.
Sporting Clays: Shoot what works the best, which could be short medium or long.
That said, if you are a competitive shooter, and your goal is to win shoots, do what the top shooters are doing. Which usually means shooting guns with 30 to 32" barrels.
What is driving the long barrel trend in SxS guns could be connected to the long barreled target guns of today. I'm not sure. I know, at least for me, the longer barrels just work better on the long complicated targets they are setting these days.
Long barrels just seem to be smoother on the long targets.
Another maybe better question could be: why the trend to long complicated targets in a game that's supposed to replicate hunting?

Sporting clays as well as FITASC has gone on to the competitive level. Thus the more complicated targets to sort of shake out the shooters into classes...

JMHO

What doesn't kill us only makes us stronger. I know, without any doubt, that my years competing in registered sporting clay competition, in the NSCA, made a better wing shot out of me in the field and marsh/swamp.

I shoot better with longer barrels, almost without exception. My dedicated A H Fox quail and woodcock gun has 28" barrels. That's very short for me. The best shooting I ever did on quail was with a 30" barreled 28 gauge. As an aside I will contribute this bit of ephemera. In 25+ years of competition I've never been outshot by anyone using 26 or 28 inch barrels. You may value that information accordingly, based on what it cost you.

I agree with the above posts, and think the SC shooting has improved my overall gun game, for sure, but no shooting game has improved my field shooting like box birds.
I'm sorry so many had to die, but it was for the greater good ;-)

Many believe that by changing guns, shot, technique, stock length or whatever is going tp greatly improve their shooting. I started with a 28" barrel 63 years ago. I found a great deal on a citori with 26' barrels and found it improved my SC shooting. I bought a 32" 686 but soon traded for a 26' Citori that the retailer said couldn't be used for SC. The best length gun barrel is what works best for you.

A "new gun" always improves one's shooting. At least that's what we tell our wives ;-)

Based on how tall the guns are in my safe, the 30" guns are no longer than the semi-autos with 26" barrels. Due to the semi-autos having the vent rib plus the top of the receiver, I believe both have about the same amount of sighting plane. Thus a 30" or 32" double is easily adjusted to if one grew up on hunting with semi-automatics. A 26" double on say doves would not feel the same. Just my two cents.