Two reasons:

1. Musk's vision for the Twitter product is theoretically and practically destroying what was great about Twitter.
   1. His dogmatic approach to free speech is resulting in a wave of hate speech underming any semblance of reasonable discourse on the platform.
   2. The pay to play model for verification ($8 for blue check) is actually undermining 1 above as the newly introduced Twitter algorithm will promote "verified" accounts to the top of your Twitter stream while suppressing the "unverified" accounts further down.
2. Musk's business strategy is unethical.
   1. Demanding employees "lick his boot" with seemingly zero input/oversite from internal committees.
   2. Cutting headcount because his failing product model in 1 above is resulting in negative future cash flow estimates.
   3. Leveraging employees from Tesla to support with Twitter.
   4. Leveraging Tesla/SpaceX equity to bolster the failing business of Twitter (keep in mind Tesla and SpaceX receive funding from the govt so he is indirectly using our tax money to pad his failure).

Given these concerns, it is reasonable for a person to simply not want to engage with Twitter especially because the value of Twitter is the content the individual user provides to it.

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edit: oh, one other major thing, Musk is a manchild rightwing memer who a lot of people find despicable on surface level actions alone. As such, simply not wanting to associate with that shit stain is sufficient for many.