Because he didn't commit treason or anything approaching it. Rather, he was responsibly doing his job, and all of his calls were staffed, communicated and coordinated with the Department of Defense along with the US intelligence community's interagency pipeline.
Milley was aware of intelligence suggesting that the Chinese believed the U.S. was preparing to attack and he feared a hair-trigger situation in which there could be miscalculation, or a preemptive strike by China in an attempt to fend this off or get ahead of it.
And concurrently there were tensions over military exercises in the South China Sea, tensions aggravated by Trump's increasingly belligerent rhetoric toward China on the campaign trail. So Milley tried to assuage these fears by reassuring China that the U.S. was stable and not gearing up for an attack on China.
The conversation after January 6 was because China and much of the rest of the world was convinced that Trump was unhinged, desperate and capable of almost anything.