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.