Because she didn't just "become asian", she went through a very involved process utilizing both magic and technology to completely recreate her. One aspect of that was putting her consciousness in the body of an already trained and hardened warrior, whose body was conditioned for fighting. I'm not sure how familiar you are with the concept of "muscle memory", but physical skills aren't something you can just program into someone's mind and have it work. Kwannon probably began training in childhood, and her body was conditioned and trained to such an intense degree that someone starting in adulthood would probably never catch up. To go along with the body, the appropriate skills and fighting knowledge were transferred mentally to Psylocke as well, making her a hardened fighting machine in mere days whereas training her from scratch to that same level would have taken years if it were even possible at all.