It's a favorite of pre-existing fans from prior editions. My suspicion is that 5e has brought in so many new players who never saw the "warlord" in those prior editions, and the concept has to be explained to them.
Just think about how this sounds to a new player: "First, conjure an image of a 'warlord'. This class has nothing to do with that. Second, it's a lot like a cleric or a bard, except that it's not magical, which is really important. Third, before this makes any sense we have to explain the concept of 'roles' such as 'support' from previous editions, even though 5e doesn't really have that..."
By then the new player has already checked the "not interested" box in the poll.