Well, in light of that then I'd have to agree that it's probably something related to a fault or limitation of the motherboard. If you CAN get a motherboard with a UEFI bios, that would be the better option so long as there is not a major difference in price because then it will stand a much better chance of offering ongoing bios updates, support for a newer and broader range of hardware and potentially continue to support newer hardware if you keep the system long enough to need for it to do so.
Otherwise, you might actually be somewhat limited to whatever is still out there and available. Obviously there are no longer any of these boards being manufactured so you are stuck with either new old stock (NOS) or used. I'd probably shoot...