[sebtron]

For OpenBSD on a server in particular, many useful tools (web server, mail server, pf...) are developed by the same team and better integrated in the base system. This means consistent documentation, behavior and syntax (e.g. for configuration).

In contrast, Linux distributions are a collection of software taken from different sources, with all the quirks that may derive from this.

(I don't want to imply that the BSDs have only advantages over Linux and not the other way round, but the OP asked specifically why _BSD over Linux_).