>In a country with complete control over the information ecosystem, how can it be that so many old people don't want to take the vaccine?
Because nobody wanted to be held responsible for any adverse reactions to the vaccine. The sordid state of medical liability culture and vaccine hesitancy in China would take too long to explain here so let's just say that the distrust runs pretty deep, and the government is understandably cautious with the vaccine program. One might even say too cautious as the vaccine came with a long list of contraindications that cover every category of geriatric illness, and it was not even approved for the 60+ age group for quite some time.
The propagandists in China did all they could to portray the vaccine as safe and effective but stopped short of actually enacting a vaccine mandate. The medics were more concerned with self-preservation, going as far as telling young couples to avoid pregnancy for 6-12 months after getting the vaccine just in case it might cripple the foetus. All of the these mixed messages only reinforced the existing suspicions that the vaccine was rushed and unsafe.
Of course none of these actually matters. Why waste political capital when the problem is effectively socialised among 1.4 billion people? Just keep covid out of China and everyone will live happily thereafter, right?
Australia and NZ, in contrast, deliberately made it very difficult to get any kind of exemption for the vaccine. Not to mention that both countries had endemic Delta variant by the time vaccination began in its earnest. Fear should not be underestimated as a motivator - ethnic Indians topped vaccination rates in multiple western countries because everybody had lost a relative to covid in India.