Because the books never had a truly calm moment that makes for a ride outside 4 things in a way that fits Hogsmeade and Hogswart(riding a hippogruff, hogwarts express, the boat ride to the cave where potter saw dumbledore die, and the threstal carts to the castle) . The London side would be where the tame rides could be as the story made the wizard city life fun and safe as there truly is nothing beside the ministry elevators and the knight bus so you could truly make up anything you really wished. Lastly its nots Universal who has to be like we can do whatever we want but you forget JK Rowling has to approve. Using the parks to expand the story can end up contradicting the story complete. Like oh yea lets go into the forbidden forest and have a tame ride when the forbidden forest was forbidden for being too dangerous which is why Draco had a fit in the first book and Potter and friends had to sneak into the forest. She is protective of her property she isn't going to greenlit something just because Universal is trying to get a family ride. You can't make a ride about the worst people and ride through Azkaban and be like lets make this tame, that entire thing is about the worst people.
Edit: Went on 3 tangents: One staying true to the story because of JK Rowling. Two, Why Azkaban can't be a tame ride. Three, why forbidden forest can't be tame.