Because only a few people used that. The usual workflow I saw before GitHub made PRs popular was: one local repo per developer (or more repo if they wished so)

Because only a few people used that. The usual workflow I saw before GitHub made PRs popular was: one local repo per developer (or more repo if they wished so), one central shared repo, push a feature branch to the central repo, somebody pulls the branch, reviews the changes and poss the merge them in the preproduction branch, or go back to the developer, some test build deploy procedure, acceptance tests, pass or go back to development, merge in the production branch, test build deploy.