I can think of two possible answers:

Not that much paleographical work has been done on the Ordinary - all the interest has been on the Propers.

Also, the manuscript sources for the Ordinary are later, as a rule, than the propers, because apparently (we think) only a few Ordinaries were sung even into the Carolingian era, and the composition of new Ordinary melodies only took off in the 11th to 12 centuries and later. This is why the melodies are oftentimes a different world - more scalar, more use of 1 3 5, etc. By this later era, the manuscripts tend not to show as much rhythmic nuance, so the neumes which one could find would not be nearly as instructive as for the Propers.

Perhaps there are other reasons as well - I don't know.

awr