Because he has no character other than saying a bunch of edgy shit about "losing" and "speed", and etc. Not to mention, the thing about him feeling "guilty" about killing Ichigo and Nigo feels forced, especially considering we don't know why he killed them, or why he joined up with Shocker in the first place, or what he even is as person. So at the end, he's just a blank slate.

And well...I know I'm quoting Azenzone from his review of All Stars New Stage on this one, but by making them blank slates, we can't really get a solid grasp on who they are as a person.

And also, Kamen Rider V3 is Sango. He always has been Sango, always will be Sango, no ifs ands or buts.