Because the .40 is a bigger round ?Imo, 40 s&w is pointless in 2014 and going forward. The reduction of recoil with 45acp and 9mm, along with the various advancements in bullet design makes it so.
Since handgun rounds are not optimal, why not choose faster and more accurate follow up shots which youre more than likely will need in a self preservation encounter?
Because my wrist does not hurt when shooting the .40 ?
Oh , and it's bigger. (I have 9mms too just wanted to answer)