Two relevant tidbits of info:
- When Sequoyah created the Cherokee writing system, he had to prove its usefulness to the Cherokee elders, in order to promote its adoption.
- Plenty societies had access to writing, but it was only used by a ruling elite and/or a cultural elite, with most of the population being illiterate.
Based on those two things, I think that the usefulness of writing only becomes useful for most people if they know how to write or have heavy contact with the ones that do it. I mean, you can remember most stuff just fine, and you can speak with other people, so what's the value of learning a weird system to put those words in material form? So it's one of those things that only appear rarely but, once it appears, it spreads fast.