Despite the massive level of popularity that the Ultra Music Festival has attained, the Miami City Commission has denied their contract and therefore barred them from returning to Bayfront Park for the next festival (scheduled for March 2019). The decision was the result of some political tension within the commission, and a very divided base of local citizens.
As reported by The Miami Herald, many downtown residents who oppose the festival's return were leaving City Hall feeling very pleased, while the festival's organizers seemed pensive, trying to decide their next play. The organizers did reportedly say that they still want to try and salvage some kind of appearance at Bayfront Park, though if they don't, it'll be the first time in almost twenty years that the festival has not taken place there.
As the Herald points out, it is possible that they could negotiate some kind of deal with a different venue, such as the Hard Rock, but that remains to be seen.
Below is an official statement from the organizers, delivered via a tweet: