I think the likely reason is they have signed a non disparagement agreement and have no avenue to publicly criticize. Additionally, quietly criticizing just means quietly getting fired, then likely still being unable to publicly criticize.
The only way to say "hey, I don't think this is right and I don't agree with what's happening here" is to publicly resign and hope that alarm bells start going off in people's heads as to why many of these folks are resigning simultaneously.
The only way to say "hey, I don't think this is right and I don't agree with what's happening here" is to publicly resign and hope that alarm bells start going off in people's heads as to why many of these folks are resigning simultaneously.