Yeah, it started as a television pilot and got canned before film ever rolled. But someone as Sony liked it enough that they attempted to resurrect it as a backdoor pilot/miniseries (like Battlestar Galactica or that Knight Rider pilot a couple of seasons ago), hoping that if it drew enough views that they could greenlight a series.
... but then, after much deliberation, they decided to just turn it into a movie. It still feels sort of like a backdoor pilot - like an expensive, two-hour long miniseries. It has television-style pacing near the end, and the whole thing feels like, "Tune in next week for the further adventures of Zombieland!"