Because she wants to

Yuka was having a hard time with Nyu, more specifically trying to keep her from removing her clothes. It started after when Kouta toke a year off from college to save up some money for them so he went overseas to the U.S as a Japanese teacher. This sadden Nyu so Yuka did her best to cheer her up, this started one day around 12:00am as Yuka was making curry. Yuka had just finished putting all the ingredients and was started the rice when she hears a crash and a whine. Their was Nyu on the floor wearing a white (now brown) shirt and blue jeans whining before smelling her arm starting to lick the curry off. Yuka hurried over to the pink haired girl looking over her making sure Nyu was unharmed before picking her up and walking to the bathroom. Their Yuka found out two problems: one Nyu had gotten curry everywhere even her underwear so she had to remove them from Nyu, and two all of Nyu clothes were in the wash and Yuka's didn't fit her so she had to let the girl stay naked at let until her clothes were done. Nyu didn't have problem being nude as her childlike mind give her zero modesty so she ate lunch naked and spent the rest of the day wearing nothing. Come the next day Yuka finds that the Diclonius was still naked waiting for her to make breakfast Yuka thought it was strange as by now Nyu can dress herself. So after making breakfast Yuka went Nyu room to get her a simple blue dress and matching underwear. Yuka dressed Nyu with some strange wines from her only to turn around to hear the sound of cloth moving. Turing around Yuka saw Nyu removing her underwear showing her untrimmed crotch to Yuka before dropping the panties with the other removed clothes. Yuka thinking to herself ( what had gotten in the girl) as she tried to get Nyu to put the clothes back on only for Nyu to back away turning into a run caused Yuka to chase after her. After three days of this Yuka gave up thinking everytime she put something on Nyu only for her to take it off or worse tear it off so she let Nyu be a home nudist thinking and hoping she'd put on clothes before going out in public. Boy was she wrong.