Because it followed a week of ignoring the issue and then another week of arresting people who were reporting on the issue. This has been an issue since December 28th, but China and the WHO are pretending it began just recently.
It's "containment" in much the same way that the Soviet Union just couldn't stop lying about Chernobyl because people miles away were melting from the inside, so they took drastic actions to pretend they were on top of things.
Those "hospitals" they are building (and selectively live streaming parts with glorious Party tunes on the background)? Those were resorts that were almost complete, but since tourism in the area is going to be dead for a decade, they're turning them into hospitals and claiming they're brand new buildings.
What is staggering to me is not that China finally moved when it realized the problem could not be swept under the rug, it's that the WHO is going along with the doublespeak of saying "all is well" but then also declaring an emergency, but don't cancel your trips to China!