Ultimately, it's probably due to the rate of encounter with this species and the evolution of the pheromone towards the most consistent and intense response from that particular species. It's likely that a more general pheromone wouldn't induce such a consistent response, since it would be a less specific and reliable cue from the ant's perspective. So if you encounter lots of different ant species, a more general pheromone with a 50% response rate could work, but if you primarily encounter this ant species, a more specific pheromone with a 90% response rate is going to confer a greater fitness advantage.
Could be due to the rate of encounter with this species vs. other species, could also be due to the consistency of response with more general vs. specific pheromones. You start with spiders with silk/pheromones that ants are a little more aggressive towards, then variance in that chemical makeup leads to fitness advantages. If the pheromone confers a strong advantage in a specific ant,