I am not sure about the other ones, but I vaguely remember that NetBSD makes driver development relatively easy, or at least one does not need to re-write or co

I am not sure about the other ones, but I vaguely remember that NetBSD makes driver development relatively easy, or at least one does not need to re-write or copy-past driver code when moving from, say, PCI to a USB-attached device. I can imagine that makes it a sensible choice for companies that build their own hardware (custom peripherals and so forth). Also, as others have pointed out, the BSD License. Some companies won't touch GPL code with a ten-foot stick.