With Linux, there was an independent need for a "pull from upstream" model, so one was built then further developed. That wasn't necessary for the BSD's, and by

With Linux, there was an independent need for a "pull from upstream" model, so one was built then further developed. That wasn't necessary for the BSD's, and by the time someone would consider rolling their own "distro" they would probably already be part of the FreeBSD project. A part of it is certainly userbase size, but the project seems to allow more competition while still being part of FreeBSD. An example, FreeBSD ships with three different firewalls. With Linux, each one would probably have built their own distro with that being the key difference between them.