Two relevant tidbits of info:

1. When Sequoyah created the Cherokee writing system, he had to prove its usefulness to the Cherokee elders, in order to promote its adoption.
2. 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.