Love can’t defend the rim. KP can. It’s about fit. KP offers outside shooting like Love, but is a far greater threat as a rim-runner and just a considerably better defensive presence on that Cavs roster.
Also, is Kyrie going to walk away from all those KP pick-and-rolls if and when Bron walks? Maybe, but it’s considerably less likely than if the Cavs stick with Love or make a short-term move like George. KP and Kyrie are the perfect modern combination and would almost certainly attract a star player in free agency at some point down the line.
It’s a deal for the present and the future for the Cavs.