Because I can only spend so much on comics per month. I specifically mentioned comics I might be interested in buying. I have to choose between Daredevil and that comic if I want to not spend more money each month. I'm sure I'll spend it on Daredevil, but that's not the case with every book that doubleships.
I'll also add that inconsistent shipping bothers me more. If a book is always going to be twice a month, I will drop another book from my pull list. If a book is often twice a month, do I keep that other book and spend slightly more the doubleship months or drop it and spend less the single ship months?