Something has confused me for a while with sheet music.  This doesn't happen that much, but it does happen.

Sometimes, a piece will be in a certain key, but the actual notes in the music are missing one of the flat or sharp notes in the key signature.

If the key signature is A major with F#/C#/G#, then I'm watching out for those special notes, and expect to see them.  Let's say this tune has F#s and C#s, but no Gs anywhere to sharpen.

In this case, why didn't the composer label it as Key of D major instead, since that would have worked too, and been less confusing?