I think it is because you are not yet pre-1700 qualified. The father you propose replacing is born before 1700 and you do not have the proper access. I understand your reasoning as you laid out above. If you would provide the sources, I am sure someone would be willing to make the corrections for you, if you ask.
You could also take the easy test to become pre-1700 qualified. I do not know your level of experience, but it does not require any in-depth knowledge be qualified. Here is the Pre-1700 Help page: