-A rainbow is caused by light reflecting off the small drops of water remaining in the atmosphere after the rain. The drops of water are not pure and contain tiny partials of dust the light is therefore reflected.
-The world before the flood was very different from the way it is in current times.
-The flood made major changes in the atmosphere. One of the things it did was to make everything a lot denser, a lot more physical.
-Before the flood the water was so fine, so pure that light would have passed right through it without making a rainbow.

(Rabbi Yonasan, Tiferet Yonasan – Noach)