We know Helevise pretty much never took the girls seriously, but it kinda makes me wonder what, at least from her point of view, makes one a good candidate to be a Godcaller? Obviously not just folding immediately when someone tells you to smoke your Halv, for one lol but
It's a rare job that requires a certain mindset. Helevise took one look at the girls and didn't see it. (she also wasn't in any way inspired to even try as she put all her teachings into Iralee, who eventually denied the career path, but Helevise still held hope she'd take her mantle)
God-callers are often solitary deer, most often in mind than in body, as they'll learn things and perspectives the rest of the populace lives ignorant of. They need to take their advice from the Sky and not the Ground, (test failed as they blindly obeyed Helevise instead of seeing the importance of their halves), which isolates them from their peers. The lifestyle of a god-caller isn't natural for deer who thrive in their social groups.
In short, you need to be a bit weird to be a god-caller. Or you need to have a kid who you can mold into one. I'm sure there are plenty of fake-callers who claim the title because it's a very cool title to have, or doom-callers, who preach senseless stuff to frighten the masses to act like their own moral compass dictates.