I think  it is alteration  of the genome

exposure to insecticides might have influence. In some areas, we have the areas with susceptibility to DDT but resistant to Deltermethrin

Both DDT and deltamethrin (a pyrethroid) target insect sodium channels so mutations in the channel (target site mutations) in response to DDT may make the mosquito also resistant to deltamethrin. However, the mechanisms of resistance are many. In Aedes aegypti there are detoxifying enzymes that are specific to DDT (DDTases) and their up-regulation may not result in resistance to anything else. That may also be the case in Aedes albopictius where we have found significant resistance to DDT but susceptibility to various pyrethroids (we did not test deltamethrin). Which specific mosquito species are you working with?

Although both have targeted site-dependent sodium channels of voltage, the pyrethroid deltamethrin metabolic resistance is associated with detoxifying enzymes: esterases and monooxygenases. In contrast metabolic resistance to the organochlorine DDT could be associated with monooxygenases and glutathione S-transferases (GST). Thus, the mosquito that is overexpressed genes associated with GST can mediate resistance to DDT but not to pyrethroids.

I agree with the previous answers and I would like to add that in the past DDT have been overused in public health and agriculture which has exerted pressure on the susceptible population (my information is the oldest pesticide still in use) allowing only resistant individuals to survive and pass their resistant to their offspring (enzymatic mechanisms/ genetic explanations apply hers); while pirethroids like deltamethrin and similar pesticides originated in crysantemum have been used more recently (applies to countries I know) and the process of resistance and enzymatic adaptation are still under development. There is reported resistance to deltamethrin specially for Aedes aegypti and albopictus.

I agree with  Eduardo Fernandez

I also agree with Eduardo F and Ishag Adam. and will still emphasize that the excessive and indiscriminate use of this pesticide paved the way for the resistance being observed today.

I do agree with Eduardo Fernandez. Mosquitoes will certainly develop resistance against pyrethroids if used unjudiciously. 

And what about KDR. Do DDT and Deltamethrin has diifferent markers for KD resistance in mosquitoes ? Some papers have mentioned that KD resistance is linked to to different mutations in mosquitoes ? Is it true ?

It is obvious that the resistance level is due to selection pressure. Studies have also shown that resistance may be multigenic and some time mutations with certain co-factors may produce the resistance phenotype. Mosquitoes with L1014F mutation genotype are resistance to DDT but may not to certain SP compounds.