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Because of the whole "master" meaning a slave owner thing that the progressive left seems to think it means and it apparently hurt enough feelings to make it a movement (default main branch on Github repos is now also "main" because of this), and then you have companies that desperately try to be hip and cool, and they do this to get social clout. The whole thing is pretty sad if you ask me, but what do I know.
There is nothing wrong with trying to use more inclusive language regardless of said reasons, and to be honest main is actually a better name anyways.