First, FreeBSD is probably the most common BSD you'd run into. Definitely has the largest user base. And of course, large user base, more developers, and usually more ports maintainers. Not always true, but true most of the time. FreeBSD has also usually focused on the server case, so a lot of the improvements are improvements where servers will see the most benefit. That sorta explains the, why FreeBSD over OpenBSD and NetBSD. I personally prefer NetBSD, but there is a solid argument for FreeBSD.
Why FreeBSD over Linux? To me, I would say it depends on what sort of engineering model you think works best. Linux is great, but it is essentially a giant orgy of a lot of independent projects that luckily work together to create a linux-based operating system. The kernel team is different from the gnu team that works on gnu libc and gnu core utils. Even with Rust core utils becoming a thing, it's still a separate team. Linux doesn't have a true notion of a 'base' system like the BSDs do. Each of the BSDs provide a 'true base' system. That means pretty much all of the code (maybe a few exceptions), but the core hot code on a freshly installed FreeBSD operating system is all owned and maintained by the FreeBSD team (this applies to NetBSD and OpenBSD and DragonflyBSD). This is the libc, the kernel, the user land utilities, basic programs and even some advanced ones (like OpenBSD's httpd or firewalls), and even the boot loaders! So it provides some sort of coherence to the platform that just isn't on linux.
Linux, and I use that a lot to, is really an amazing anomaly in software. That in the most minimal distribution, you really have tons of software from teams that may have little to no interaction with each other, can some how all be compiled together to work.
Addition: Wanted to make an addition to the difference in engineering decisions. The BSDs in general make clear distinctions between what is provided by the system and what is from third parties. An example of this is reading through some of the file system hierarchy stuff in FreeBSD. Yes, Linux technically has a file system hierarchy and things are supposed to have a designated place, but it's much more Wild West on linux. FreeBSD and the FreeBSD community generally conform to the standards defined in `hier` pretty well.
Why FreeBSD over Linux? To me, I would say it depends on what sort of engineering model you think works best. Linux is great, but it is essentially a giant orgy of a lot of independent projects that luckily work together to create a linux-based operating system. The kernel team is different from the gnu team that works on gnu libc and gnu core utils. Even with Rust core utils becoming a thing, it's still a separate team. Linux doesn't have a true notion of a 'base' system like the BSDs do. Each of the BSDs provide a 'true base' system. That means pretty much all of the code (maybe a few exceptions), but the core hot code on a freshly installed FreeBSD operating system is all owned and maintained by the FreeBSD team (this applies to NetBSD and OpenBSD and DragonflyBSD). This is the libc, the kernel, the user land utilities, basic programs and even some advanced ones (like OpenBSD's httpd or firewalls), and even the boot loaders! So it provides some sort of coherence to the platform that just isn't on linux.
Linux, and I use that a lot to, is really an amazing anomaly in software. That in the most minimal distribution, you really have tons of software from teams that may have little to no interaction with each other, can some how all be compiled together to work.
Addition: Wanted to make an addition to the difference in engineering decisions. The BSDs in general make clear distinctions between what is provided by the system and what is from third parties. An example of this is reading through some of the file system hierarchy stuff in FreeBSD. Yes, Linux technically has a file system hierarchy and things are supposed to have a designated place, but it's much more Wild West on linux. FreeBSD and the FreeBSD community generally conform to the standards defined in `hier` pretty well.