Void magic killing otherwise immortal zerg leaders is a lazy plot device
The Overmind and cerebrates are explained as being effectively immortal. Unless killed using void powers they will instantly "reincarnate" their body. There are a few problems with this plot device. Firstly, it's a lazy plot device and was obviously shoehorned in later in development. Tassadar wasn't initially intended to divebomb Overmind, this was suggested by the design team to add an epic climax to the script. I can't be sure, but I suspect the vulnerability to void power was added as part of this because the presentation is clunky (see my second point). The SC1 manual never once implies that the Overmind or cerebrates were immortal or could only be killed by void powers. Secondly, the introduction of this plot point is very clunky and involves characters reading ahead in the script rather than coming to reasonable conclusions. In order: Overmind tells us that cerebrates are immortal Zeratul permakills Zasz Overmind says only dark templar can kill cerebrates ...