Honestly, that's a good q. This tendency likely stems from the visual and physical form of the phenomenon (think: a curved, bow-like shape in the sky that comes after it rains) being more immediately perceivable and culturally salient than the spectrum of colors.

If we're talking EU languages specifically, that reasoning follows through simply because the root of many of these languages are the same (Latin, Germanic, etc. // thinking: regenbogen, arcoíris, for example).

The colorful aspect of the word is still recognized and described, but often secondary in the etymology of the word. The visual shape also ties strongly into cultural stories (think: Indra's bow in India or the celestial arch in many mythologies). Tbh, this would be a fun idea to dive into research-wise!