I could see Brian happily transferring the Captain Britain mantle to Elizabeth for the sole reason that he's had enough of the superheroics and just wants to settle down and take care of his family.
It could be THAT simple a reason. (Especially as we know from their history as kids that Elizabeth was always the one yearning for grand adventure while Brian was more the studious, bookworm type. He could see this as his chance to not have the responsibility of overseeing an entire dimension anymore).