Because academia is a funny place with a unique pecking order, and the specific degree you hold matters, sometimes a lot. MSN, DNP, and other types of ARNPs are professional degrees. You get them in order to perform specific jobs/tasks. You’ll have clinicals, make connections, and likely get hired for a job before or shortly after the credential is complete. But inside academia, a DNP is not considered the equivalent of a PhD because the DNP did not write a dissertation and has not made a significant contribution to the field of study.
So, in academia, the gold standard is a PhD, and in many places, from whence that PhD came can make a difference. PhDs in academia, whether teaching or in research, need to publish. Who you know gets your name on the articles that get into the journals that lead to rank, promotion, and tenure… But, it’s kind of that way for all fields, not just nursing.