Gonna guess that it looks too busy compared to the simpler and easier-to-understand layout. The static pictures also convey the level better than this.
The recent leaks have provided answers to something I've been puzzling over for a while now. Boss Bass slowly became the modern frill-less Big Cheep Cheep, while many of its original characteristics were given to Bubba/Cheep Chomp, a separate enemy itself based on Blurp from Super Mario World. Always fun learning the intricacies behind developmental changes!