Exactly the same reason we have "console wars" (and why people find it so hard to get out of a con deal) - no-one wants to admit they are wrong, therefore they will defend their choice.
With coding (and consoles) it's not only about not being wrong, but being right-er than others, to justify your investment in time in your toolset (or few hundred bucks in your choice of console).
Therefore any competing technology is frowned upon.
It obviously does not help that language demand sometimes is superficially and in extreme measures inflated not by what it does, but how many non-technical people demand it, either for direct project work or recruitment agencies hiring to match their client requests.
Not a surprise then that programmers feel the need to publicly make every other language or framework look absolutely useless and incompetent.
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