Hmmm, seems like a lack of guts to me. I mean comon we had "Re-animator" so why not bring zombie sex to TV!
That said, I think the lack of revealing where the Zombies come from is a cop out, and one common to the sub-genere. Simply put the entire zombie apocolypse scenario is very difficult to justify, and a lot of creators try and justify around establishing any kind of solid mythology due to all of the questions people rightfully raise about the feasibility of
things turning out like this.
Once you determine where the zombies come from, you then have to explain how the plague spread so far and so fast. A "patient zero" scenario won't work in a case like this. Unlike a lot of stories about regular diseases, a zombie virus has something you can kill through conventional means to prevent the spread, it doesn't travel through the air or anything to infect huge numbers of people.
This of course leaves them with supernatural explanations, or setting things up so the virus WAS airbourne and the survivors happen to be those who are immune (this also explaining how the infrastructure was decimated) but of course it wrecks the whole "if you get bit, you get infected" logic that generates a lot of the drama in this setting.
I haven't read the comics where they might explain things to a nessicary degree, however as far as the TV show goes I kind of feel that if they don't establish their mythology that's actually going to hurt the show in the long run, despite it's initial success.
As it is, "Walking Dead" is already full of "WTF" moments, as I for one can't see how the heck Zombies overrun a bloody tank that isn't crewed by morons (and not every tank crew is going to be full of morons). Not properly handling military response, or saying that the military is "overrun in two weeks" or whatever, even if it was somewhat decimated in the initial outbreak is REALLY pushing credability since the military will form up under crisis, establishing a new structure based on whatever ranking people survive, and truthfully with the right tactics and weapons I don't think any number of zombies could overrun them.
I mean, yes there could be an apocolypse, yes the military could be overrun in specific places, but your not likely to see the entire thing wiped out, especially not as quickly as we see in these scenarios. You'd have tons of hold outs, and I'd imagine a lot of those "safe zones" that people head for and always find destroyed when they get there, actually wouldn't be overrun unless the people running it were a bunch of mouth breathers, and not every outpost is going to be full of retards.