Because in this sentence nuut is not directly describing the noun inside the noun phrase; it comes after the linking verb is.
Compare:
So the position changes depending on the structure of the sentence.
This is a very common learner question.
In Die kar is nuut, the adjective comes after is, so it stays in its basic form:
But when the adjective comes before the noun, it often changes form:
So:
This basic contrast is worth memorizing, because it happens with many Afrikaans adjectives.
You say:
Afrikaans typically uses double nie for standard negation:
So:
This double-negation pattern is one of the best-known features of Afrikaans grammar.
The normal, neutral word order is:
Die kar is nuut.
That is the most natural way to say it in ordinary speech.
You can move things around for emphasis in some contexts, but that creates a more marked or literary effect. For a beginner, it is best to learn and use the standard pattern:
subject + is + adjective