Bhagwan Sri Krishna had killed Narkasura , who was the father of Bhagadatta. So he was an enemy of Bhagwan Sri Krishna. Bhagwan Sri Krishna was the main guide, philosopher, friend and strategist of the Pandavas. They always seeked advice from him. So this alienated Bhagadatta from the Pandavas. I think this was the main reason why inspite of being the friend of Devraj Indra and Samrat Pandu , he fought for the Kauravas.
The second reason is contradicting itself , read ahead to see the contradiction.
KMG does’nt mention Arjuna’s victory over Bhagdatta but Bori does mention it.Arjuna had fought against Bhagadatta when he had gone to conquer the kingdoms in the Northern direction. They had fought against one another for eight days and finally Bhagadatta had accepted his defeat.
Now here comes the contradiction. At first BORI mentions that Bhagadatta was welcomed and treated with honour .
But then again Duryodhana tells Dhritrashtra that Bhagdatta was not allowed to enter the gate.
So if we take Duryodhana’s words to be true here , then this too maybe a reason but I feel the first to be the main reason. The second reason even contradicts itself, first it says that Bhagadatta was welcomed and then it says that Bhagadatta was stopped at the gates.
Anyways I believe the first to be the main one. If someone can clear the confusion about the second part , it would be great.