It just came as the path of least resistance. We upgraded all our devloper pcs a few years ago, so we had the old desktops just laying around. When the need came for a new server, it was easiest just to use the old desktops we had on hand.
Also another reason, it is much easier and cheaper to maintain standard desktop parts.
Hardware availablility isn't amazing here in Czechia, so sticking to more mainstream CPUs and form factors makes it much easier to fix them when something goes awry.