That's because they cowtowed to JKR and Warner Brothers. They saw Potter as their only chance to pretty up the parks they had neglected for nearly a decade to finally sell them (their goal for the 10 years they rand the resort). They had no desire to run theme parks, and cut costs at every corner throughout the years, including selling off valuable expansion land.
Potter turned out great because Uinversal Creative, the WB designers, and contractors were actually given some room to play. It was still a relatively cheap project with a ton of reused infrastructure. There's a reason Diagon will blow it out of the water, and a reason why the Hollywood and Japanese versions will be significant improvements.