What is so powerful about it? Genesis dispersed an entire nebula, a phenomenon likely to be trillions of times bigger than a mere star. Also, a trilithium torpedo hitting a single planet would probably accomplish nothing but a local case of poisoning - while Genesis hitting a planet is known to be destructive, and Genesis hitting a star might be pretty spectacular as well.
Given how quickly the battle was over in WOK, I'd say that the Mutara Nebula was pretty small for a Nebula. It was pretty near the Regula station but we didn't even see it in any shots until near the end when they fly into it; if it was really a big one we'd have seen it in the background. Doesn't seem all that big compared to a Sun.
The torpedo wipes out entire solar systems in one usage, Genesis is a one-at-a-time matter. And I don't think it would work on a Star, unless the energy of the torpedo is superior to that of a star.
Frankly how a tiny torpedo like Genesis could have the energy to create a self-propagating effect never made much sense to me, but TOS' logic was always "Look, the tech just does what it does. We're never going to give any explanations over any of this, just accept it or go away."