In the expression V必要がある / V必要はない, which I came across in my textbook, I would expect that both use the particle が. What is the reason why は is used only in the negative case?

This is the source:

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  • 1
    Highly related: japanese.stackexchange.com/questions/70188/… Nov 10 '19 at 14:25
  • Thanks for your answers. @Naruto, "So how did the は come to be used anywhere? This is probably part of an historical process of trivialization and loss of meaning. It seems to me that originally, the pattern ~ではない was used to only mark a strong contrast of the entire predication to something else (e.g. something that was said or implied before by someone else), but later became more and more popularized until what originally was a contrast marker became an almost necessary feature of the negative form." is the only part that tries to answer my question, but I find it too vague...
    – jarmanso7
    Nov 10 '19 at 17:45
  • Better than nothing anyway. Thanks.
    – jarmanso7
    Nov 10 '19 at 17:45
  • @l'électeur, thank you very much. I think the は in yout linked question is the same case than mine. However, the provided answer explains the how but, unfortunately, it says nothing as for the why.
    – jarmanso7
    Nov 10 '19 at 17:48

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