Important note mentioned here is that sqlite doesn't accept external contributions and has a very small team. The Fossil folks and the sqlite folks are also essentially the same team. Fossil is a boutique VCS which essentially only exists for sqlite.
...and that also has certain consequences for Fossil's design and workflow; namely, an aversion to the "rebase"-style development [1]. A project with a handful of known contributors could reasonably use Fossil; for a project with dozens of contributors, which also wanted to be able to accept "drive-by" contributions, Fossil would be unworkable.
