Most answers address the major/minor capitalization, but there's one vital reason, not yet discussed in this thread, for always capitalizing the tonic:
Imagine we're talking about the key of "a minor." If we don't capitalize that tonic, a certain subset of readers are destined to have some confusion (however momentary) when they read:
With the appearance of a minor, the sound of a minor triad is no longer unexpected.
As such, we capitalize the tonic A, even if it's in minor, to prevent any confusion between the tonic pitch "A" and the indefinite article "a":
With the appearance of A minor, the sound of a minor triad is no longer unexpected.