The hotel industry just didn't see the millennials coming. The business practices for many hotels involved selling a room at a reasonably low rate, and then making a profit from serving breakfast, offering room service, providing a minibar and a whole raft of other add-ons. Millennials have a habit of refusing all those offers - they just want a bed to sleep in at night, and beyond that, they probably won't even be in the room at any other time.
There's another threat to the hotel industry which is beginning to bite, and that's Airbnb. No hotel can offer someone the comforts of home - but someone else's home can. Airbnb spaces are frequently offered by millennials to other millennials, and all the money for temporary accommodation stays in Millennial hands. Large hotel chains are still trying to work out what to do about this.