Xolair is intended for repeated use to reduce the risk of allergic reactions and is not approved for the immediate emergency treatment of allergic reactions, including anaphylaxis.
Epi Pens are for immediate emergency response for allergic reactions, they aren't preventative in any way.
Epi Pens are basically just a shot of adrenaline, they don't actually act directly on your immune system at all, it's basically just sends an override message to your body to forget about whatever else it's doing and focus on opening up your airways, pumping blood, or whatever else you might need in order to, for example, run from a lion.
Which buys you some time, but you're still probably going to need some treatment or monitoring once the adrenaline wears off, and you're probably going to feel like shit afterwards, and epinephrine isn't without it's own pretty significant risks and side effects.