I remember one theory saying Iris became alcoholic. which I agree with. The other one that I wasn't sure of, but was possible until the promo trailer for s5e4 was that Nora just preferred her dad over her mom. One reason is they have more in common. And the other reason is Nora finally gets to spend time with her Dad. I think Iris abandoning Nora works for both theories. Nora shares everything with her father and Iris doesn't want to be reminded that Barry is gone. Who knows..,?