He wants to go somewhere with more of a role, a place that will work around his skills (ball in his hands, shooting) and hide his weakness (man to man defense). They fit his style. He wouldn't get that at UK. Gonzaga also has success with transfers like him.
Another reason is that he's very, very close to his family and Gonzaga is about 400 miles away (from Portland, OR) instead of cross country. I'm sure that played a part.
There's also the fact that he's friends with some Gonzaga players. A high school friends committed to play there this past year. Plus he just finished playing with the Canadian national team and hanging out with Gonzaga's point gaurd.