Evans is a pretty bad 3-point shooter (28% this year), but they were playing off him, giving him a pretty open look at a straightaway 3.
Cousins wasn't open at all. There wasn't really a lot of time for him to make any kind of move in the post, if he got the ball it was going to be some kind of contested shot. As noted, he only half-heartedly put up his hand for the ball, the Thunder defender was right on him.