Because governments shouldn’t be allowed to just wash their hands of any responsibility to a citizen, just because they don’t like their views, regardless of how extreme and vile those views maybe.
We have judicial systems for a reason. If someone joins a terrorist organisation, you arrest them and allow the justice system to determine the consequences. Allow governments to strip citizenships is effectively a mechanism to allow governments to avoid due process and their own judicial systems. Those are never healthy behaviours in any democracy.
Societies should deal with their own citizens properly, not just strip them of citizenship and declare them someone else’s problem. I have no idea why anyone would believe making it effectively impossible to ever leave, except by dying, terrorist organisation would be a good idea. That must ensure all members of terrorist organisations literally have nothing else to live for. I don’t know about you, but I don’t want to ensure that all terrorist go out with a bang.
