yellow ranger
 

why they let aiesha leave

She was living in America at the time not Communist China, she had the freedom to come and go as she wished. Now if it was Communist China she could have been forced to stay.

As for the real world answer, the actress wanted out of the show for whatever reason.

yellow ranger
 

i like aiesha better than tanya becuse she was one of the classic rangers

Not much is known about Karen Ashley's (Aishia) departure. But for whatever reason the actor left the show.