Oh, but if it was your homework, I can guess the teacher wanted to say that "nihil" was more common and more literary... Well, even in modern languages, we use contractions rather in spoken (or not formal) language than in written one...
But "nil" appears even in well-known Latin proverbs, e.g.
"Homo sum, humani nihil alienum a me esse puto"
or: "Homo sum, humani nil a me alienum puto"