I think there are a lot of people who feel that 3D World was rather reductive, and step back for the series, myself included. It's still well designed and a great game, albeit a lesser experience than most 3D Mario's.
It was just boring! It was the most Mario Mario (Mario...) except that Mario is known for innovation. Its the same problem I have with New Super Mario Bros (other than the original, which was actually innovative).
Not to say Yooka Laylee is better.