I believe this question has been asked and answered in this forum
on more than one occasion.

If an address is listed as an owner on more than approximately 50
lists, the List Management page will not offer the drop-down menu
of lists owned.

The LISTSERV postmaster is the de facto owner of every list on the
server, so if the server has more than approximately 50 lists, an
individual who logs in using a LISTSERV postmaster address will not
get the drop-down list of lists owned.

--
Paul Russell
Senior System Administrator
University of Notre Dame