I think I've figured why.
The characters of BtVS and Angel are wonderfully complex creatures; that's why they feel so real to us. They are also, without exception, flawed. Buffy is insecure. Willow is shy. Xander is geeky. Giles is repressed. Cordelia is bitchy. Angel is guilt-ridden.
And where does this leave Riley? Well, Riley is that least recognisable of cultural stereotypes; the all-American hero boy. He's nice. He's sweet. He's honest. He's loving. He's charming. He's tough. He's clever. He's spotless, whiter than white, and that gives him all the character dynamics of a piece of cheese.
A knight in shining armour might be what Buffy needed in the aftermath of Angel, but the fact is there's no place for Mr. Perfect in Sunnydale. The trouble is, he could annoy us just by being there; simply because he was the archetypal hero, always doing the right thing, no hesitation. And that cheapens the glory of the _true_ heroes, the ones who _are_ flawed, and rise above it to make us love them.
Remember how it felt when Buffy went to face the Master, terrified she was going to die? Willow, putting on the resolve-face and doing the soul restoration spell from her hospital bed? Even the little things, like Xander rising above their petty differences and paying for Cordy's prom dress.
Riley could never truly win our hearts, because he had got nothing to do it with. Our heroes can grow and change and surpass themselves, but Riley was already sitting up there on the top of that pedestal. The only direction he could go from where he already stood was down. And boy did he do that.
Everybody has a favourite characters, their own personal heroes. We even root for the dark end of the spectrum, down there in the shadows with Spike. We hang on in there, grasping eagerly for those little tiny flashes of hope, those little moments that let you believe that maybe, just maybe, there's something in there worth saving.
You couldn't do that with Riley. What's to pray for? We can't care about his dreams or his future. You can sit there with white knuckles wondering if Spike's going to rise above his dark origins and help the good guys, if Xander can overcome petty jealousies and take the mature course, if Giles can break out of the self-imposed walls he lives behind and reach out to the people he loves. With Riley, we *just don't care*. His internal struggles just weren't big enough to register on the size of angst-o-meter you need for a show like BtVS. And when they did get big, it was too late for most of us to come to care about them.
You know what? Everybody always said fans took against Riley because after Angel, nobody would be good enough. But they tried so hard to make Riley good enough that people hated him instead. He was white-bread hero boy - and white bread is bland.
In any lesser show, we might have be able to accept Riley without question. But here, in amongst the powerful, three-dimensional heroes who hold our attention, he stood out in the worst possible way.
There was *nothing wrong* with Marc Blucas' acting. There was *nothing wrong* with the idea of the character, or the way he was written. But he never reached out to me, because there was nothing there for me to identify with. He was charming and likeable, but I never saw anything of myself in him.
Captain Cardboard, indeed.