Because it's complicated and most people don't need that complexity at all. For some reason a lot of people happily jumped on the band wagon when this was released and started writing scripts to make it more bearable.
If you have multiple versions of your codebase that you need to maintain for a longer time to justify those release branches, gitflow might be for you. IMO GitLab flow is much more applicable for most people: https://about.gitlab.com/2014/09/29/gitlab-flow/
Because it creates a large amount of needless busywork.
It's much easier to develop on master and/or on feature branches that can be cleanly merged in to master, and then tag master (or create branches) once a release is hit.
