Its because it has cool name and uses the same bolt as 556 in an AR.
And people didn't used to understand how to get .39 to run well in an AR so it got a bad name in the past. .
No offence intended, but I have yet to have anyone I know who runs one hard.... Like something I would depend on with my life claim any success with it claim that the new AR 47's make the grade.
Plinking and a fun gun: sure.
OP:
There's a lot of fads.
The ability of the .300 to be loaded with 556 brass and .30 bullets is a big factor.
The ability to be chambered in a semi (AR) SUCCESSFULLY (reliability wise) as well as a manual is big.
I bought a Remington model 7 before I bought a .300 AR upper.
The fact is that every AR that's NOT an AR15 has.... Long term parts issues.
I remember an AR10 that took HK91 mags. Great idea.
Now 15+ years later.... Good luck getting parts.
The AR10 is GETTING there in "Standardization" but isn't there yet.
So, crappy AR options in x39, long term parts issues, desire for a semi (AK's don't suppress well.) Needing different mags vs using 556 etc
I'm .sure you know that everything but the .300 barrel works with 556 with Ar's
Etc.
Also, 10 years ago the subsonic 7.62x39 offerings were crappy killers.
I was actually planning on starting a thread about good "killing ammo" projectiles that were .311, and performed well at subsonic speeds.
I recently picked up a Ruger American in 7.62x39 to go with my Model 7 in .300 blk, and intend on doing some compare and contrast.
I REALLY LIKE it's ability to use mini 30 mags (paid $15 for a 20 rounder today)
If your JUST using a manual, and just for fun.... X39
If your playing like me or have lots of different guns.... Meh
But if you don't have 40+ different types of ammo, commonality is a good thing. (If I were to use an AR pistol it would be in .300 or 9mm, NOT 556.) I
I HAVE 556 pistols, but as toys.
If there some good subsonic projectiles for the x39 I can see the (my) .300 guns going unused or going away.
Otherwise:
Have you HEARD a suppressed .300?
Add that to the above.... And that's why
$0.02, YMMV.