Dio could have done what he did to Kakyoin to Jotaro, and he would have been the victor. Why didn't he do that? I'm pretty sure Dio was smart enough to have thought to do that.
DIO was too afraid that Jotaro was going to be another "Jonathan" in which he'd win. The magnets that Jotaro put on himself and he made DIO think that Jotaro was just another "weak" Joestar, in which he got cocky, and decided to go for the fatal blow with the stop sign. (For dramatic purposes.) DIO misses, Jotaro beats the shit out of DIO, yada yada yada, DIO takes Jotaro more seriously, but he goes back to being cocky after he achieves his "greatest high."