It's because covid also has evolved to be less lethal with higher transmission rate. But this new variant that's come out may be a bit more lethal and they are not sure so the policy is kind of going back and forth.The emphasis has always been on how Chinese health care lags behind US, Japan, etc… therefore zero Covid must be kept to prevent millions of death in densely populated Chinese cities and thus overwhelm the healthcare system. That was the justification for Zero Covid. Read the last 20 pages.
Now, all of sudden it has “all the data it needs” to take care of “exacerbated Covid”. Not much has changed except Xi decided to loosen zero Covid after the party Congress.
It's hard to open up during the winter time as other disease flare as well