This seems to have been a condition of his employment. Though this appears odd now, it was common practice for a successor to marry the daughter of his predecessor, in all walks of life. It was common, for instance, for a promising (and lucky?) apprentice to marry his master’s daughter.
A generation later Handel visited with his friend Matthesson (in 1703). Buxtehude was 66 and perhaps wishing to retire, so Mattheson was considered as his possible successor. But when they offered him the job it transpired Buxtehude’s daughter was part of the package. Mattheson suddenly lost interest.