This occured to me a few years back, but has anyone else considered that Izsha may have been the "cerebrate" of Kerrigan's Brood War campaign? I mean, they haven't had any problem filling our nameless character's shoes before (ie, Liberty, Artanis).
Had it been present at the QoB's crowning achievements, it makes more sense why it's remained loyal to her as opposed to the other Brood mothers, which were made much later...