Thank you all for nice thread of discussion. There is some light on that. I shall explain here in most simple and most convincing way.
Actually, it is quite easy to understand why it is called as di-a-tonic scale. It basically means a scale with TWO (di) tonic centers.
The one tonic (root) note is, (say) 'C' in C-Maj. scale.The second tonic (root) note is (say) 'A' as root note in A-min. scale.
Thus same set of notes creates TWO (di) centers forming major and minor scale hence called diatonic scale.
