Probably for the same reasons brought up for the demon emperor vs Devil Shintai discussion. Getting a lot of health means you're a good tank, but enough mortals with guns can eventually kill you if you let them keep trying.As far as I can see Hardened Devil Body is the only one called out as being able to take multiple times which would imply the others are not, that said I'm not sure why the author thought armor was less balanced than health.
Get enough soak, like say from taking demon armor 4 times, building with good stamina, and taking a soak charm, then you could stand there while a dozen guys with assault rifles unload at you to no effect at essence 2.