Many reasons. Some of which are:
1. Rhinoplasty is a complex operation which is done differently by different surgeons. A suboptimal result does not always indicate what exactly was done to the nose. Some surgeons are not comfortable with unpleasant intraoperative findings which limit their options. (Similar to building contractors who prefer to build new homes rather than remodel homes and be subjected to surprise findings in the walls)
2. Some suboptimal results may be impossible to correct. Especially in multiple revision cases. Very few surgeons are willing to put themselves in a situation where they deliver another poor result to a patient who is already stressed over the appearance of his nose.
3. Some surgeons may look at your nose and regard your deformity as minimal in nature. As such they are fully aware that revising your nose may result in a worse result rather than in an improvement and as such would rather pass you to someone else.
4. An unhappy patient, despite your best efforts, may remain unhappy. As such, he may decide to sue BOTH you and his previous Plastic surgeon(s). Many Plastic surgeons are not willing to assume such risks.