Because, most likely, Disney doesn't own the theme park rights to the property.* T.L. Travers is publicly known to have despised Disney...she hesitantly allowed them to make the movie, which she hated. She most certainly wouldn't have given them the theme park rights. She would have made J.K. Rowling look like a pushover!
*Note
Disney doesn't even own the stage rights for the property. Disney was only able to produce it because Cameron Macintosh, who actually OWNS the stage rights, didn't think it would be accepted without the Disney score, but the storyline was completely re-worked for the West End ("Scary Poppins) and totally ruined for Broadway.