The simple answer
By convention, one writes in
B major rather than
Cb major. The exception being when
Cb major better expresses key-relationships.
But some parts are in sharps at the same time others are in flats. Why?
An answer that begs the question
The bassoon is notated in
B major, because it is playing with the strings, which are notated in
B major. The real question is, "why are the strings notated in
Unfortunately, in the absence of an explanation by Tchaikovsky or his copyist, one can only speculate. Thus...
There are generally two reasons why a passage is notated enharmonically. Either
- It better expresses the key relationships; or
- It's easier to read.
The case against "key relationships"
In this case, notating in
Cb major would better express the relationships: the passage is sandwiched between two
Ab major sections. Modulation by minor third would generally make more sense than modulation by augmented second. There's a possibility Tchaikovsky was considering this passage in terms of
B being the key of the leading tone, but this seems unlikely since the
C minor passages are relatively distant.
The case for "ease of reading"
I think it's more likely Tchaikovsky felt it would be easier to read. For one thing, up to this point, the score has been notated primarily in flats. Tchaikovsky may have felt it would make the modulation easier to see/understand if he switched to sharps. The preceding passages were in
C minor, where no additional flats were needed, and
Ab major, which required only the addition of
Db. That pitch,
Db/C# does not occur in the
Cb/B major passage. Tchaikovsky may have considered its presence in the preceding
Ab major section and then absence in the
Cb/B major section could lead to confusion. In other words, writing in
Cb major would involve acts both of inclusion (
Cb, Fb, Gb) and exclusion (
Db); whereas, writing in
B major involved only inclusion (
B-natural, D#, E-natural, F#, G#, A#).
Why not notate the other winds in B major?
Here I'd go with ease of reading. Since the winds are playing accompaniment chords and relatively few different pitches within each part, easier to just keep them in flats.
So why are the horns notated in both sharps and flats?
Best guess...because they're the vertical mid-point in the score. So Horn 2, vertically nearer the strings in the score, is notated in sharps, while Horn 1, vertically nearer the winds, is notated in flats: a visual aid to the conductor.