See Dick Continue to Run, Continued (3) - S2-E2
Evil Dick: Stay and witness my moment of glory, as I impregnate the entire population of Ohio with my demon progeny.
Tommy: Wait, wait. Even the men?
Evil Dick: Okay, not the men.
Sally: What about little girls?
Evil Dick: Mmm, no, not them.
Tommy: What about elderly women?
Evil Dick: I don't think so.
Harry: What about women who are already pregnant?
Evil Dick: Oh, shut up, all of you! Okay, so apparently I won't be impregnating the entire population of Ohio. But all fertile women of child-bearing age who are not currently pregnant, and that's a lot!
Answer: It is not know for sure but some of the reasons could be the number of walkers. This was not just happening in one place but the entire world. If this was just happening in Atlanta then troops and equipment could be sent from other places to help, but since this was happening everywhere overwhelmed troops had no help. This wold does not have zombie movies. They have no idea how to handle this and have to learn everything like only head shot, bites kill and everyone turns.