I've mentioned this like a broken record, but a likely factor is that Stipe was and is still a part-time firefighter throughout his career. This may be a reason why Stipe hasn't evolved much during his heavyweight career ex: he has no submission wins, his kicking game is very basic.
No, I am not talking about him unlocking his Croatian potential and becoming Cro-Cop 2.0, but he could have added kicks like push kicks, more head kicks, to keep DC at bay. In many of his fights he does a good job of controlling distance to land his punches, but he looked lazy in maintaining distance everytime DC would come forward in this fight.
As a competent amateur boxer, he could've circled more to stay away from DC when he would come forward. Simply standing in front of him and banging obviously wasn't the best option.
As the other poster mentioned, it seemed that he didn't see DC as the potential threat that he truly is.