What makes you think kemba is better on defense?
Kyrie is bigger, younger, more efficient offensively by a pretty significant margin, and is objectively a better defender.
Kyrie is better on offense and was more efficient. Although he was allowed to be more efficient due to not being the only player the other team needs to worry about on offense. Kyrie had a lot more help around him then Kemba did.
Based on watching them both play. Kryie often takes plays off and gets blown by a lot. Kyrie lost in the playoffs because he tried to be the primary defender on players and 1 on 1 got destroyed by them. If you want to go by stats Kemba has a career higher DWS, DBPM, and the same Defensive rating. Although I am not a fan of defensive stats for individuals. Kyrie has been on much better teams that helps with those stats.
Celtics were a worse team with Kyrie than they were with out him. Irving had a negative net rating this year in the playoffs.