It depends on the hosts cancellation policy. Did you attempt to change/cancel the reservation last minute? For example, if a guest contacted me on the day of check-in and wanted to move the reservation back a week, it's likely that I wouldn't let them or would charge a fee. In this case, this theoretical guest essentially held the original dates they booked and now nobody else will book them. If I let them change to a week later, I'm just out that income I would have gotten otherwise.
I don't know your situation, but I'm going to guess that it's something along these lines. The host is probably following their booking policy that you did agree to.