Why are some prefecture names written in hiragana on license plates? I noticed a license plate with hiragana characters written on it instead of Kanji. I had assumed that all prefecture names would be written in Kanji. Thank you.

No prefecture names are written in Hiragana officially. As you note they are all written in Kanji.

License plates are not from the prefecture but designated cities (some of which ARE written in hiragana (usually due a dispute about what the name a newly formed city after a merger, e.g. ))

For scooter liscence plates, any city/ward/village/town can issue them so the city/ward/village/town name appears. If that happens to be Hiragana then hiragana is displayed on the plate.