Airlines have a high "spoilage" rate compared to most other industries. Once a flight leaves the gate, an empty seat is lost and non-revenue producing.
Aircraft is expensive and requires huge capital outlays. The return on investment can be different than planned.
Large workforces spread over large geographic areas, including international points, require continual communication and monitoring. This can be exacerbated during operational irregularities, such as bad weather.
While the business climate can change quickly, airlines have difficulty making quick schedule and aircraft changes due to leases, staffing commitments and other factors.