We looked into both when writing an open-source admin interface for NestJS. We went for bootstrap because:

- Nobody likes bootstrap, but everybody knows bootstrap (good for an open-source lib) - Better browser compatibility - Better accessibility - Plenty of UI widgets built on top of bootstrap (we don't use them, but people using the lib could) - About as big as each other

No solid reasons honestly, other than personal preference. Bulma just looks better out of the box to me.