She released a video of the name change last year, stating that it was more of a business decision for easier recognition.
I believe it was more to 'bury' the previous name "Kalani", because of the seemingly negative connotations the name had, especially with the ******** and ****** stuff.
Lastly, it would be because her management team (her mom and dad) not only wanted to wipe the slate clean for her entertainment career, but because the public could easily mispronounce her last name "Kalani" (correct pronunciation Kuh-Law-Knee), where some people have probably pronounced it like 'Kal-Annie', 'Kal-Lain-Knee', 'Kal-Luh-Nye' etc., the same reason why her first name is spelled and legally changed to "Kylin", whereas her birth name was "Kailynn" or "Kailyn".