Because the primary meaning of 'constancy' is moral or emotional - faithful, dependable - and that is not what's required here. I think a verb phrase like this works better.

Of the three adjectives I suggest, 'immutable' works best in noun form here - 'the immutablity of human nature'. You can't have 'the fixedness or fixed nature of human nature' (argh!) and as I've said, I think 'constancy' doesn't quite work in this context.

Using a verb phrase also helps you avoid a near-repetition of the previous 'uniformity of human nature' so in my view makes the text read better.

Hope that helps.

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Note added at 37 mins (2010-01-22 12:00:43 GMT)
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