Notably, it is mentioned that the 7-bit code is developed "in anticipation of" ISO requesting such a code, and we see in the addenda attached at the end of the document that ISO began to develop 8-bit codes extending the base 7-bit code shortly after it was published.

So, it seems that ASCII was kept to 7 bits primarily so "extended ASCII" sets could exist, with additional characters for various purposes (such as other languages, but also for things like mathematical symbols).