What it means
でなくてなんだろう expresses a strong rhetorical conviction that something must be or can only be the noun or na-adjective stated before it. Literally, it asks what the subject could be if it were not that thing, implying that no other explanation is possible. The pattern is typically used in written, reflective, or emphatic statements rather than in neutral questions.
Usage note
This expression has a rhetorical and somewhat literary tone. The polite counterpart でなくてなんでしょう is available, but it remains emphatic rather than becoming a neutral information-seeking question.
How to use it
これほどの成果は努力の結晶でなくてなんでしょう。
What else could such an achievement be but the fruit of hard work?
彼の判断は無責任でなくてなんでしょう。
What else could his decision be but irresponsible?
これほどの成果は努力の結晶でなくてなんだろう。
What else could such an achievement be but the fruit of hard work?
彼の判断は無責任でなくてなんだろう。
What else could his decision be but irresponsible?