Because numbers are the best way to navigate a linear sequence. Numbering each note also allows you to see the numberical distance between two notes, thus making it easy to see their interval relationship. For example, the distance between a 1 note and an 8 note is +7…..so +7 is the interval.
Using letters is not as usefull!
Soon, I will be doing tests with groups of first time keyboard players comparing Traditional to NN. I can’t wait!
Jason