Dramatic effect
Drama is conflict. Look at the root of the word "protagonist" and you find "agony."
Which Apollo mission resulted in the biggest movie, after all?
I know what you mean though. When Red Planet and Mission to Mars came out almost back-to-back a few years ago, I figured between the two of them they'd take NASA down.
I had hoped that James Cameron's Mars Reference Design would result in a movie, but alas...
Well, there's always this. (Loved those books.)