In my experience it all comes down to performance and tunability. Our personal preferences are just that, personal.
Linux has vsyscalls which makes it performant for the typical LAMP stack. If you profile a PHP Zend framework "hello world" app it makes 20k+ calls to gettimeofday(), which is very slow on FreeBSD in comparison. Throw a couple hundred requests/s and you saturate the CPU for no good reason.
FreeBSD needs business support in order to maintain funding, and be a relevant competitor to Linux. If a developer were to pick a popular application stack like node and identify significant performance gains it would make waves.