Two reasons actually. First, the .380 is much better with today's ammo, but it's still a very small cartridge, and not in the same league as the larger three you referenced. Secondly, 9mm ammo (practice or carry) is much easier to locate.
In a skinny sub-compact, I think the 9mm is the balancing point for power, control, capacity and gun longevity. Just my .02c.
I love my G42, but will pick up a G43 as soon as possible due to the cost/availability of 9mm ammo vs .380. The G42 won't be going anywhere.
