At that point, they might as well have put up a giant neon sign that said "We Don't Care Anymore". There was no "fan-service with no thought behind it" angle to thank for that storyline. No one asked for that pairing. Nooooo one. There was no interior logic to it at all. It was just "Well, the main gay couple has split. Let's pair one of them with the other recurring gay character we have." "Who?" "Karofsky." "Oh, so we're finally going to make Kurtofsky happen?" "No no no. Not Kurt. Blaine!" "Oooooh, game-changer!"
The only time those two even interacted on screen before getting together was when Karofsky shoved him into a wall after suggesting he was closeted. It's not like they could've been like "Oh, the chemistry!"