A couple of guesses - the still-lingering fallout from Mirrors, great songs on the record in the classic BOC crypto-mysterioso style but nothing really Top-40 friendly (like BfY), lots of competition in the marketplace, if I remember correctly Creem (previously staunch allies) gave CE a none-too positive review including some harsh words about EB (pre-Herbal Life though), the Rolling Stone Record Guide rates it lower than Mirrors and barely above RbN and CN (!).
If you peruse the 'regular' reviews of CE on-line (amazon, Steve Hoffman, etc.), it seems that the those who've heard it really like it, it's just that not enough did/have.
That's an understatement Buck! From the observation of being in record store management at the time the promotional handling by Columbia was horrible. Number 1 I think they should have released Deadline as the 1st single while trying to get Monsters in heavy rotation at the radio stations. Similar to what they did with FOUO. WBCN Boston had Joan Crawford, Burnin' and VOTPW all in rotation at the same time. WQBK Albany had Joan Crawford, Burnin' and the title cut in rotation. Great promotion that Cultosaurus didn't get. Monsters had that "new" original sound like a Joan Crawford. Add Martin Birch to the mix and like you said, "should have been a big record". I would've followed up with Black Blade and Divine Wind. I got to see the Black and Blue Tour in Pittsburgh and BÖC closed the show with a bang. Dio was cool with the Sabs. I was up front and saw attitude from other members. Kind of like Ritchie Blackmore on the Mirrors Tour. From all the forums, FB and social media through the years Cultosaurus is always mentioned in fan favorites. Now I'm in the mood for a listen! Original vinyl or remastered Box?!