The problem is that the vast majority of folks, not to mention scientific evidence, do NOT find DE to be a wonder cure. A new person gets on here and sees all sorts of posts ravingf about DE and says to themselves "WOW! Now I know how to prevent my birds from ever having mite or lice issues." Those of us who are at best not convinced of its effectiveness could just sit back and think "they'll learn, why should I stick my neck out and contradict a common opinion, even if I believe it to be wrong?" Or we could respond with our opinions.
DE may help prevent an infestation. If WILL NOT cure one.
Winter is often the time that mites/lice thrive; however, different pests tend to occur and become an issue during different seasons, not to mention some are more common to one area of the country than to others. There are a number of different insecticides/miticides other than sevin that are effective at killing these external parasites. Alternating between different ones that are effective against the bugs common to your area helps prevent them from developing a resistance. There are some productas that either kill eggs or contain an insect growth regulator that prevents non-adults from maturing and repopulating.