When creating a new design system you need to cover at least:

1. CSS architecture, making all the components inter-compatible with each other (very complex).

2. Accessibility.

3. Browser compatibility and responsiveness/mobile capabilities.

3. What components are you going to provide?

4. Documentation for the team using it.

5. Fixing all the bugs in it.

6. The _actual design_.

Bootstrap pretty much gives you 1-6. It makes decisions that are easy to criticise if you've got the resources of a big tech company behind you, but for most people, customising Bootstrap will let them focus on #7 and get all the rest pretty much done for them.

There are alternatives, taking any of the major CSS frameworks like this that has customisation built in will work, Foundation is another good one, but Bootstrap has a lot of the mindshare.