From a budget perspective, Disney/ABC is delaying hiring anybody because it's so much cheaper and safer filling guest co-hosts than to start someone at a salary equivalent to Kelly and likely see little if no gain in ratings. Suppose the pairing doesn't work in the ratings, or Kelly doesn't get along with the person. Anyone famous would want atleast Kelly's salary. Kelly likely wouldn't pleased in an arrangement with the new male co-host making more than her too. Anyone not so famous who would take a low salary might not bring any uptick in ratings, and if he did, he'd ask for more salary in a year or two likely leading to issue there.
Anyways, the current safe arrangement works as long as Kelly + co-host formula is fine in the ratings...