1) You ran out of time to write tests.
This might be the least bad reason. You can probably talk your way out of this by explaining what tests you would have written if you had the time.
2) You don't know how to write tests.
This shows that you have only worked at places the don't do testing and that you haven't taken the initiative to learn about testing on your own.
3) You know how to write tests, but you don't think they are needed.
You could probably tell the interviewer why tests were not important for the particular exercise. But I have never seen a take home exercise that doesn't require a basic algorithm and some tricky conditional business logic. Both of which would benefit from unit tests.
4) The code you wrote isn't testable.
This is probably the worst reason. Everything is in one or two giant functions and you can't inject dependencies for testing without refactoring everything. This will frighten your interviewer.