Because if disabled toilets were always unlocked I can guarantee they would be occupied by hundreds of able bodied people, mainly women. You always see a queue for women's toilets. They would soon abuse the disabled toilet. Making it increasingly more difficult for a disabled person to then go to the toilet.
Don't forget it's not just a toilet. It's a larger space for pad changing, bag changing and other necessities.
Keeping these toilets under lock and key so its strictly for the disabled is more than reasonable
As a parent of a severely disabled child it's one of very few 'luxuries' we get. No chance I could take my son into a regular toilet without prying eyes.
If anything having it under lock and key allows me to keep my sons humanity