It is because each word doesn't sound like the other words so if it cuts out part of the word for interference or background noise you know 'lta' was delta and 'vo' was bravo and not some other words.

Experts and multilingual people were involved in choosing the words that could be pronounced clearly by people who spoke different languages, the words were each unique, and they have been consistent across many decades. The current list is the survivor of multiple attempts to do the same thing!