Hm...I'm not sure, kind of hard to tell. But I can guess. I think a lot of the revolutionary aspects of Grand Theft Auto IV were reiterated upon and made better, and pretty quickly, which led to a lot of the free praise games usually get for being innovative to sour over the years. I think the novelty of having a next generation GTA quickly deteriorated, at least in my circle of friends, Red Dead Redemption was WAY more popular, and I think most people who play Red Dead Redemption nowadays probably love it more than GTA 4 players do that game. GTA 5 being the massive success it was is kind of like a small cherry on top probably, as the legacy that GTA IV inherited for being the first "modern" GTA has become almost irrelevant. But I could just be bullshiting, who knows.
However, how overrated is the game truly when even IGN agrees it's overrated? And moreover, how much longer will we be completely reactionary and require time to process what is "overrated"? I don't find GTA V that great either, but a lot of Rockstar fans do. Will that also have a similar fate? Probably not, but if it didn't have multiplayer ... most likely.