I think a few things play into this. Scott was there from the start and we've actually seen him and Kourtney be a couple and have fun. Scott has a personality unlike Tristan. Khloe and Tristan never seemed to be a great match and I just can't see what she sees on him.
Their show and their social media presence weren't as big and while Kourtney put up with a lot, she never blamed others or went off on them on social media. The way Khloe tries to call out people through obnoxious quotes also doesn't help. I think Kourtney seemed quite removed from their relationship for a very long time while Khloe seems to very much want Tristan and likes to sing his praises in between the cheating scandals.