This remark is really unkind. D3 precedes Observable, and the development path is exactly what someone would take if they wanted to make data visualization accessible - create a base library, and later opinionated tools to work with it.

The harshest way to interpret unavailability of regular JS tutorials should be that the primary authors don't have enough bandwidth or the inclination to work on it, and the community hasn't picked up the slack to make it more accessible to beginners. The authors deserve nothing other than gratitude.