Why comma before "not"?

Here's a good guide: https://owl.purdue.edu/owl/general_writing/punctuation/commas/extended_rules_for_commas.html Point 8 is what best applies here.

Think of it as two sentences: - People aren't trying to sell busyness as a virtue anymore. - They aren't even trying to sell busyness as a virtue to themselves. This sentence combines these two points into one sentence. The comma separates the main point of the sentence ("People aren't trying to sell busyness as a virtue") from the adverbial phrase that adds extra information ("not even to themselves").

It's a parenthetical. It doesn't change the meaning of the sentence and the sentence is grammatically complete without it, but it provides further context. "A decade later," is also a parenthetical. The independent clause in this sentence is, "people aren’t trying to sell busyness as a virtue anymore"

The comma before the “not” adds extra emphasis to what’s being said after

Because, when you read the sentence aloud, you are likely to pause at that point. If you try to say a long sentence without pausing you tend to run out of breath and it becomes difficult to understand because it get garbled and there's no hiatus to allow time to comprehend what has been said.