Many advice will be grate...
Most Helpful Guy
There could be hundreds of reasons, or it could be he is just lazy, I say this with the expression in mind that "the devil you know is better than the one you don't" and no I'm no saying your a devil, just that he already has a basis for that relationship, and while he may not have gotten everything right, the things he did get right are now known, thus those are things he doesn't have to learn with you, which is where the lazy part for him comes in. Relationships take work for both parties, and it sounds to me like he isn't willing to work on something new and possibly better. You should feel encouraged that you are not going to have to try and live up or perhaps down to what he had become accustomed to with his ex.
Most Helpful Girl
Well, as you've stated, he wants to give it another go with his ex. Keep in mind it's not you. You didn't do anything wrong. Something in his last relationship was not finalized. That is clearly between him and her. She could've done something stupid and begged for him to come back, sometimes you miss people when you aren't around them, or something else. At least he told you, right? He isn't leading you on, so that's a bonus. My only concern is if he and her breakup again and he wants to talk to you again. Just be careful. You're not a backup plan. If that situation arises, be upfront and tell him how you feel.