Because Karin completely recovered from that and even came back all lovey-dovey to Sasuke. She didn't feel any pain, she didn't cry. She accepted it all.
Sakura cried because of Sasuke and suffered 'til the very end for him. It wouldn't be fair to have him end up with her rather than with the girl who didn't really mind the murder attempt.
I'm just going by some sasusaku logic, but one could argue that he wasn't his true self that's why sakura couldn't accept him. the sasuke in chapter 698 was the one sakura fell in love with. Though that's my interpretation of it , I always saw the bonds in team 7 deeper then any other in the manga, so maybe that's why i can comprehend sasuke and sakura together more so then sasukarin.