In addition to most of the other answers given, I wish to point out that the 380ACP ammo is made on the same equipment as the 9mm Parabellum ammo, usually in January, by setting case length a bit shorter and loading to lower power levels. This means guessing at demand, which has risen sharply in recent years, and shop-time competition with the 9mm sales which are much larger.
Add to this that the public has been given new high quality 380 guns lately, the round is marginally adequate for home and personal defense, it is fun to shoot as there is little recoil which means popcorn-like shooting sessions at ranges which consume ammo dramatically without wearing on shooters.
The 380 is easy to reload, but typical shooters seldom are the "reloader type of folks". This is unfortunate, as most 380 ammo is 40% reduced in power to protect old poorly built import guns, some of which are still out there in use. HOWEVER, the Hornady Handloading Manual publishes full power SAAMI spec loads for modern 380's in good condition. This includes specs for using Hodgdon Universal Clays powder (not Universal, which is something else) for ammo with almost NO MUZZLE FLASH for use in darkened rooms without blinding the shooter! The lighter JHP bullets have reduced wall penetration, particularly the Sierra ones, with adequate penetration of heavy clothing.