If the kanji for "nani" was not there, but just the hiragana, it would mean "I think that's because maybe it's impossible". But you could translate it as simply "I think that may be impossible".
For instance, when you say "ame nan desu", you are saying something like "It's because it's raining". The "nan da/desu" after a na-adjective or noun, works as an explanation.
February 27, 2013 at 3:16am