I think it's more a case of almost every shooter having a 9mm pistol, the vast majority of those being people who don't reload and don't bother to learn. Savings of 50% are achieved pretty easily even with 9mm. Also, reloaders tend to branch out a bit into other calibers.
and 9mm is the cheapest you can reload ..even with premium components ..115gr FMJ bullets in 9mm ...are costing my about $ 0.09 each or around
$ 4.50 for a box of 50 ...if you like to reload ( and I do ). I'm almost out of 9mm range ammo ....so I'm going to load up about 4,000 rds this weekend in 4 or 5 hours ...no big deal...
I shoot plated in 9mm, and lead in .380 ACP, so I can do better with .380. Funny because .380s are significantly more expensive in the store, twice as much or more as 9mm in some cases.