Those above me are fairly obvious reasons, but also the fact that it took Jessie three tries to even get to the GF. To get there you just need 5 ribbons, which Jessie barely just made with the last contest held. Now James would need 8 badges. Sure they can make animé only badges for him, and maybe one or two game badges if they revisit a town with a gym, but that still means that he would need a whole lot of more skills and the such, or he will probably get some but not enough by the time the league roles by.
They could integrate this in the story as well: double battle gym battle anyone? Ash and Jameson (his obvious alterego) vs something like Tate and Liza? Lol, if his alterego would be Jameson, that would mean he's going to win the league, or at least beat Ash! xD
But that is random speculation. For now, we know that James is good at exploring competitions and lets not forget he has taken interest in Pokéringer. Which I like more then contest at the moment, just don't let something evolve in it everytime and I'll be fine with this either coexisting with contests or replacing it.