She likes to wear them

Blake Belladonna was someone who had seen a lot in her relatively short lifespan. She had seen all kinds of discrimination due to her status as a Faunus, she had taken part in all kinds of protests, and had taken hits from the man she thought she loved.

After the Fall of Beacon, Blake needed to get away from the **** and the shame she felt from failing to save Yang from losing her arm to a violent assailant. The only place she knew to go to was her family's house in Menagerie. While reuniting with her parents, things seemed tense at first, but they still took her in after everything.

Blake was still traumatized from what she had been through, so her mother, Kali, asked her if there was anything she could do to help her feel more comfortable. Blake's idea was very unorthodox, to say the least. Because of everything she had dealt with, Blake had started to long for a simpler time in her life, and admitted to her mother that she wanted to be treated more like a baby, diapers and everything. Kali was more than eager to have her little kitten back, so she agreed.

So for the next few months, Blake wore more babyish clothing, played with toys in her free time, and hasn't even used the bathroom unless she was taking a bath. She didn't know when she had been nearly as happy as she was now.

One morning, Blake was sleeping peacefully in her crib, wearing a bright purple kitten onesie and a thick diaper.