Environmentalists should be against using these items on any day keeping the pollution problems in mind. As far as my knowledge goes there are three problems in this usage of firecrackers. The first and the foremost problem is a fire hazard. The chemical fire is very hot and the flame temperature will be very high. So the body will get cooked and recovering from that burning is very difficult. The patient will see the hell on earth.
The second hazard will be air pollution with effluents(Gaseous effluents ) from these firecrackers. They will release many carbonaceous gases which are very hazardous in nature.
The third hazard is property loss. By any chance fire falls on a thatched house especially in villages will cause a lot of property loss. Even tiled houses can also catch fire. These three causes lot of loss to human life and environment. That is why many people are against this usage. It should not be on Deepavali day. But on anyday.
Similarly, everyone know crackers are bad, each city or place will take its time to come to it's senses. I'm sure Delhi is highlighted mostly because its impact to humans, there are children who have lost part of the lung function that is not going to comeback. The environmentalists are giving importance to many things, it's the people who have to listen to them.