Why is tro thanh not acceptable here?
It was said elsewhere that "trở thành" should be used with nouns and "trở nên" with adjectives, by this rule with "famous" you should use "trở nên".
Probably because they didn't care to put it in lol.
But are the two always interchangeable?
For the most part yes though when you become + an emotion, it's typically trở nên. "Tôi trở nên giận" sounds better than "tôi trở thành giận".