Why is WYSIWYP diatonic while most other Alternative Notations are chromatic?
The fundamental issues with Traditional Notation stem from the fact that it is a seven-degree diatonic scale system with a workaround to implement a chromatic twelve-degree system. That workaround is the use of key signatures. As a result, most Alternative Notations implement complete redesigns of the Traditional system with ones based on a chromatic scale and staff. For some examples, take a look at these designs on the Music Notation Modernization Association (MNMA) website gallery.
Before I get started, here is a preface to this commentary.
While chromatic designs solve the issue of key signatures, I think the next question to be answered is how easily are they read by beginning students compared to the WYSIWYP diatonic design? There are four components to this question to examine:
Identifying notes on the staff
Mapping notes on the staff to the keyboard
Tradeoffs and choices
There are pros and cons for every Alternative Notation design. But as I explained on the MNMA wiki, there is no one size fits all solution. Thus, there should be choices. Players' own personal preferences, experience level, and other factors will influence how they respond to different designs. Chromatics may appeal to a lot of players. My preference is a diatonic solution.