They needed a bigger name; That's all.
I love 24 dearly, and Jack Bauer. Without him, I think we can all agree, it won't be the same. However, casting a almost nobody actor is a HUGE mistake. They should of thrown some money at a bigger TV actor or a B-movie actor to show that their taking this seriously and to provide the audience with some familiarity.
Wesley Snipes
Clark Gregg
Justin Theroux
Nic Cage (Kinda joking, kinda not)
Idris Elba (Would of been badass, but his career is on the rise)
Philip Winchester or Sullivan Stapleton (Strike Back)
David Morrissey
Lennie James
Or, y'know, they could of just make a Tony-centric 24.