When a pesticide is sprayed on a population of insects all insects do not get killed but few of them survive give reason?

Poisons are rated by LD50, the dose that kills 50% of a population. The dose can be increased to an LD90, LD99 or whatever but you can never guarantee that the dose will kill them all, as some individuals are more resistant to a specific poison than others are.

To get the last few highly resistant individuals may require a dose many orders of magnitude larger than the LD50.