Why is Kuroko in a wheelchair?
Kuroko is unable to participate in the Daihasei festival due to her injuries from the Remnants incident, and is in a wheelchair during the course of events, escorted by Uiharu Kazari.
Is Haikyuu better than Kuroko?
Unless you are like me who is new to sports. Personally I like Haikyuu! More than KnB. Though I like a character in KnB for a long time (I still do), but story-wise I think Haikyuu is better.