Izuna in his prime got defeated by Tobirama.
Why the hell would he do that? He was moments away from creating the perfect world where Izuna could have lived as well as anything he could possibly want happening.
because Madara was concerned about something more important and that's bringing the peace to the Ninja world so he had no time to play with his little brother.
Oh so he wanted more than one wish granted.
Izuna didn't know he had FTG