Among cockroaches, there sometimes arose specimens with particularly developed nervous systems, whose "intelligence" was slightly higher than that of their kin. However, the life force of cockroaches was robust enough by itself, as they had acquired the most suitable bodies for survival millions of years ago. Their bodies, being very small, and their nervous systems, being too simple, meant that even if their intelligence doubled, it wouldn't significantly impact their behaviors.
This kind of "higher intelligence" did not aid in improving their survivability, so it wouldn't become a dominant gene and spread throughout the species. There might be some "particularly clever" cockroaches, but this intelligence would not spread or evolve further.
For most species, evolution proceeded in this way. This was also the answer to the question, "Despite having a significantly advantageous brain, why is it that only humans have evolved intelligence?"
