Scales in modal versus tonal music?

Question

When tonal music replaced modal music, why were 6 of the 8 scales (modes) used in modal music abandoned in favor of only two, major and minor? Superficially this would look like a reduction in the options available to composers and therefore appears difficult to comprehend. Was there a reason why this needed to be done, or was it just an accident of history?

Answer 1

The major scale, or ionian scale, is the only diatonic scale to have both a major chord on its fifth degree and this tritone between its third and seventh degrees. This move from V or V7 to I is fundamental to tonal music. The minor scale evolved from the aeolian mode. First the seventh degree (third of the chord on the fifth degree) was raised to reproduce the leading tone of the major scale. Then the sixth degree was raised to avoid the augmented second between the sixth and raised seventh degrees. This apparent reduction in the number of scales allowed for a lot of harmonic and chromatic explorations in "exchange".