Why does the Italian word "cielo" has that I before the E?
It is pronounced /ˈt͡ʃɛ.lo/.
It makes sense for that to be the pronunciation as "ci" before another vowel makes a /t͡ʃ/ sound, but that is useful for A, O and U only, with E it already makes a /t͡ʃ/ sound.
If the answer is etymology, then why out of all etymologies this specific one was kept in Italian when it has a pretty phonemic spelling?