I think a lot of the commercial appeal is 1 "Thus Spake Zarathustra" is a cool name, and (2) there's a market who want to say that they have read the philosopher who said "God is dead."
No doubt the people who are specialists or work hard to understand it eventually figure out what he is saying, but I personally don't know any educated layman who can credibly say that they have read "Thus Spake Zarathustra" and understand what it is about.
There's a coolness factor to things like "what doesn't kill us makes us stronger" that's about as far as most people get.
Which is sort if in contrast with Kant whose writing style comes across to most people as not cool but impenetrable.