I'd say that Linux's popularity tend to translate to technical superiority, which then contributes to continued popularity.
Take something like Docker. Because Linux is popular, it was initially developed for Linux. And because Docker runs (best) on Linux, you get more deployments of Linux, and hence whoever makes the next big thing is more likely to develop it for Linux.
The end result is you have an OS that scales from smartphones to supercomputers, and so one needs quite a good reason to replace it.
> I'd say that Linux's popularity tend to translate to technical superiority
Linux isn't exactly technically superior to other OS kernels, and it's definitely lacking in innovation.
You cite Docker as an example, but that's a shining example of Linux taking major innovations from other OSes and copying it badly. BSDs had jails and Solaris had zones long before Linux got containers, and whereas those were considered security features on other OSes, Docker containers are not seen as improving the security of systems.
Other features like this exist too. Linux has refused to allow better IPC mechanisms such as what Android to be upstreamed, and the Linux replacement for select/poll is generally considered to be the worst of the bunch.
Another consideration is that both smartphones and supercomputers tend to use lots of modifications to Linux not present in desktop kernels. Android, as mentioned above, uses a different IPC mechanism, while supercomputer applications rely a lot on libraries that bypass kernels because scaling to highly parallel 100,000-core systems requires breaking POSIX a fair amount (particularly the filesystem semantics).
Linux is superior in hardware support and the amount of software Linux can run, which it not trivial, but beyond that FreeBSD and Linux are about equal in performance and capabilities when it comes to being a server. Also I would say that containers(jails) on FreeBSD is better than containers on Linux in terms of security. FreeBSD also has a more robust COW filesystem(ZFS) than Linux. The Linux crowd will get btrfs or an equivalent filesystem battle tested in the future because they are smart, but ZFS is currently the more battle tested filesystem.
Also when Netflix's open connect team finally open sources TLS sendfile than mainline FreeBSD will have much better file serving performance than Linux does. The FreeBSD downstream fork running on open connect appliances was doing 100Gbps of TLS encrypted traffic last year and I believe are doing much higher now[1].
Finally for me when I first got into Unix and Unix like operating systems I found the huge amount of Linux distros to be hard when it came to documentation. Generic Unix commands were pretty much the same across distros, but each one had a different package manager and different filesystem layouts, and different ways of upgrading, etc. With FreeBSD you just look up FreeBSD directions and don't have to worry about differences between distros.
Take something like Docker. Because Linux is popular, it was initially developed for Linux. And because Docker runs (best) on Linux, you get more deployments of Linux, and hence whoever makes the next big thing is more likely to develop it for Linux.
The end result is you have an OS that scales from smartphones to supercomputers, and so one needs quite a good reason to replace it.