I know a good portion of the middle-aged female demographic of the show's fandom had a big crush on Alex/McGarrett.

As dumb as it sounds, they might have been jealous of the Catherine character, and preferred McGarrett ending up in a bromance with Danno, because then they could still have their fantasies about him without thinking about Catherine.

Just a theory. I don't know. I'm not female, so I can't say how they think. I did see a weird obsession with Alex on some blogs (all run by women over 40), which went way beyond just liking the show.