One could turn the tables around and wonder why people would chose Linux over BSD.
To me the biggest advantage is that BSD is fully integrated (it's a full system, not just a kernel) and there are compatibility guarantees. For example a binary built for 8.0 will stay compatible with the whole 8.x series.
> BSD is fully integrated (it's a full system, not just a kernel) and there are compatibility guarantees.
That's true of Debian, too, though. We can disagree on which project does a better job at it, but they're both full systems, compatibility tested and with long-term support for stable releases.
FreeBSD and Debian are integrated in different ways. In Debian, you have a single packaging system which integrates kernel, libraries, and applications, but each of those is developed independently. In FreeBSD, the kernel and base system are developed together -- but we have two packaging systems, one for the base system and one for 3rd party applications.
Basically, Debian integrates everything, but only loosely, while FreeBSD has tighter integration, but only of a smaller subset.
It's not true in the same way. Debian and GNU are two separate projects for instance, but Debian relies on the GNU userland. Same goes with the kernel obviously, and numerous other projects. The BSD projects however (for the most part) do all their own code, all under a single source tree.
The merits of each method are up for debate, however they are pretty distinct.
The extent of integration imo depends on what kind of system you're running. For some systems, especially minimalist ones, I agree the BSDs tend to be more integrated, though they do still heavily depend on third-party stuff (the compiler, whether LLVM or GCC, being a big one).
However, once you start adding some additional packages, Debian tends to do the integration-testing more pervasively across the entire userland, "Debianizing" packages so that they all store things in the standard locations, are started/stopped using the same methods, follow the same configuration policies, etc. Once you get out into ports, the BSDs tend to give you more of an as-is set of 3rd-party packages, and are pretty lax about integration testing.
> (the compiler, whether LLVM or GCC, being a big one).
Although I think the effort is misguided, they are working pretty hard on that one. There is some considerable effort going on to get pcc as the default compiler for the BSDs, specifically so that they don't have to rely on another upstream (and so they don't have to use a GPL'd compiler).
However, once you start considering obviously 3rd party packages, then not even windows could be considered a single project piece of software.
Might also be worth noting that Debian's disregard of the upstream (compared to say Fedora, which makes an effort to get patches into the upstream before using them), has gotten them into trouble on a few occasions.
The most prominent in recent time being their absolutely braindead patching of OpenSSL which dramatically reduced the entropy in key generation, which would have never made it through the peer review process of the upstream... Debian likes to think that they are the upstream of the projects they use, but unlike the BSDs they really are not.
