Why are so many Jazz Tunes in Flat Keys?

A good indicator in the favor of flat keys would be that some of the horns associated with jazz, trumpet and saxophone, are transposing instruments.

Flat keys are in general more comfortable on these horns. The trumpet and the tenor sax are in Bb – when they play the note C- it sounds a whole step down (Bb). If they were to play a tune in the concert key of A it would put them in the key of B – not a great key with 5 sharps. The alto and baritone sax are Eb instruments. If they played a tune in the concert of A it would put them in F#, also not a great key (for anybody for that matter). On the other hand the concert key of Eb would put the Bb instruments in F and the Eb horns in C – much easier keys. 

For guitar players interested in learning jazz standards it is a good idea to get used to playing in flat keys.

The most common being: F, Bb, Eb, and Ab major (and their relative minor keys).  The good news is that guitar players don’t deal with playing a transposing instrument!