Branching by cloning was copied from bitkeeper. It was also early git's only branching mechanism. If you listen to Linus's talk when he introduced git at Google, you'll hear him conflate "branch" with "clone" because that's what he was thinking of at the time.
[1] Sadly the popular hginit.com seems dead, this was my first introduction to Mercurial. https://web.archive.org/web/20180722012242/http://hginit.com...
[2] https://stevelosh.com/blog/2009/08/a-guide-to-branching-in-m...