I'm not sure how accurate this is, but one thing I have read is that ancient writing was in some ways used as an aid to memory, rather than a means of communica

I'm not sure how accurate this is, but one thing I have read is that ancient writing was in some ways used as an aid to memory, rather than a means of communication. Imagine you have your cultural sagas and songs that are memorized and passed down from generation to generation. When writing arises, you use it as a way to make it easier to recite. You might not really care much about the "missing" vowels and you save parchment. Once you start down this path, you might go quite far before it occurs to someone to add vowels.