Private classes generally require a lot more dedication compared to public open classes. In public classes you can come and go as you please but with private classes the teacher is giving up his time, so it requires some degree of dedication from students.
It’s also poor adab to keep missing private classes unnecessarily as the teacher is giving up their time for this and in addition to this a person must think; they don’t miss work and important appointments for anything. Seeking ilm must be given the same rights and even more if one wants to be successful.
Obviously if there is a legitimate excuse and is a one off then that’s fine but as for regularly missing classes, this is not beneficial for the student or the teacher.