I think it’s because that repo is from 7 years ago, when browser support[1][2] for components wasn’t as widespread or comprehensive.

[1] See the history section of https://en.m.wikipedia.org/wiki/Web_Components

[2] https://caniuse.com/?search=web%20components

> Why not use Web Components?

If you check out his examples (e.g. clock), you will notice that he is using web components.