That’s like saying in 1935 “why would we want nuclear power when it entails everybody on earth being irradiated and dying of cancers?”
As it stands, most jurisdictions haven't legalized assisted suicide and the ones that have don't count cryonics as a means of such. That means that, when faced with a condition that entails a slow and agonizing death (particularly one which entails neurodegeneration - which includes probably most terminal illnesses, due to the injury to brain cells such illnesses often entail), there's no option to freeze oneself before that process happens; instead, one must endure that slow decline until one "dies naturally", then get frozen in that already-injured state. And then what? Repairing the damaged parts of the brain won't do much to recover the data permanently lost in the process; would one still be the same person?
Even if that legal obstacle gets "fixed", and you're legally allowed to cryogenically "kill" yourself, at what point do you do so? Too early, and you miss out on things in your own normal lifetime; too late, and you run into the same irreversible damage described above. All the while there is zero guarantee that you'll ever be thawed out however many centuries later, zero guarantee that you'll be able to live a meaningful life even if the procedure succeeds - and that's assuming cryonics is already perfected, i.e. ruling out permanent information loss from the freezing/thawing itself.
Not when survival only entails increased and prolonged suffering.