Why do automotive radar devices work only in 24 and 77-79 Ghz bands?
From around 55 to 65 GHz, there is a strong absorption band due to oxygen in the atmosphere. This area may not really be usable. The peak is around 15 dB/km. But the rest of the region from 24 to 79 GHz seems like it would be usable apart from government regulations.

The manufacturers probably do not want to use different bands in every country, so if a spectrum area is not available in just one important market, they will probably avoid it in all markets. Not sure if that has an effect in this case, but it can in other areas.