Why have pesticide producers failed to produce new mosquito-killing insecticides in recent years?

The answer is the problem of resistance. Mosquitos have proven to be resilient to pesticides treatment hence becoming more dangerous and harder to eradicate. This is due to the fact that the pesticides are inducing mutation in their genes hence giving them resistance. This is why no-pesticide-control of mosquitos, such as the use of nets, is becoming the best option in mosquito control.