Maybe because before he went loco he was the single most kickass character on the show? He was the most stable and had a great outlook on things. He wasn't pompous like Tendou and wasn't annoying like Kagami, it seemed like he actually deserved to be a rider. I never thought he was boring at all, he really brought the show back to life, for me, Kabuto was getting boring and felt like the same thing over and over again before he showed up.
Of course none of that would mean a thing if he wasn't cast right. Hidenori Tokuyama was the perfect choice for a character like Yaguruma, he can play a confident character without looking like an ass. His little mannerisms made the character all the better, it really seems like he put his own spin on Yaguruma. Then he lost it.
As Kick Hopper, when he was first introduced, he was a pretty cool character. He was being built up to be this kickass super evil series ending bad guy, he even made Kageyama seem cool. Lately though, he's been made much more cartoony than necessary. Like I've said in the main thread, if he had been reintroduced sooner then having him "stray from the path" every now and then wouldn't seem like that much of a problem. But whatever, we're stuck with him as is and I can only hope he'll get better, without Hidenori Tokuyama, I doubt I'd be able to stand him at all.