"why not go all the way, and drop the parens?" because no one found something that is simpler and shorter than just using parens. I-expressions are made up of invisible characters and my feeling is that not a lot of people who write lisp likes any sort of invisibility in their software.

For me, using parens is not a deal-breaker, as it's a relatively simple to use for dividing up things. Beats using invisible characters or many different characters.