Oda just wanted all the Yonkou to be under Xebec, but this flashback is just proving that WB should never been here. He is completely out of place. Even now Oda doesn't know what to do with him, group him with BM and Kaidou, make him look better, but he can't participate with Garp and Roger.
Even WB's reason for being here was straight wack, Shakky? Notice me?
And
@Shiroyru might be on to something, Xebec would have probably be far more of a "villain" if WB wasn't under him, in fact I'm willing to bet his initial plan was to be more of a villain than what we got.