He was deemed medically unfit to fight, probably from the drastic weight cut he was trying to do. Now Khabib isn't fighting either...the whole fight is off.
https://www.mmafighting.com/2018/4/6/17205942/max-holloway-out-of-ufc-223-main-event-against-khabib-nurmagomedov
https://www.mmafighting.com/2018/4/6/17205942/max-holloway-out-of-ufc-223-main-event-against-khabib-nurmagomedov