Here's why:
* "Soir" already implies the evening or nighttime.
* "Nuit" specifically refers to the night.
Therefore, using both words together is unnecessary.
If you're trying to express the idea of "evening" or "night," you would use either "soir" or "nuit" depending on the context.
For example:
* "Je vais me coucher ce soir." (I'm going to bed tonight.)
* "La nuit est tombée." (Night has fallen.)