More straightforward explanation is that the candidate is perfectly fine for the role so they don't want to send a rejection email, but then they have another c

More straightforward explanation is that the candidate is perfectly fine for the role so they don't want to send a rejection email, but then they have another candidate or idea for the role so they want to wait and see how that pans out first. A month later, they've forgotten they've still got a candidate or two to say no to, or figure you're probably not waiting on tenterhooks for an answer.