Poor marketing and a lack of any sort of DRM probably killed FO sales. Not that I like DRMs, but I do seem to remember Diablo taking a CD key. CD keys, although easily cracked by pirates, are generally not cracked by the general public. I may be wrong, but it is a plausible explanation as to why Diablo sold more. The other, of course, could be cross platforming. They did have a version for Playstation I believe. Not sure how many units of that contributed to their big sales #s.