There's a lot of information in a traditional western score that cannot be easily represented in a pianoroll, at least not losslessly.

Considering them as alternate views of the same data model gets problematic when the composer uses the full bag of tricks that score notation allows (notably repeats, but also the problem of representing tuplets correctly when a pianoroll can offer no clues about how to structure them). So for example, the user can create a set of notes in the pianoroll that will never be played correctly by anyone reading the score; the user can create dynamics in the score that cannot be correctly presented in the pianoroll version.

I'm not saying it isn't possible to do an MVC-style system with two different views of the same data model - it clearly is. It's just moving between the two views is not lossless, and moving between the two controllers (i.e. editing) is not equivalent.