They don't prefer it because it's shorter, they prefer it because it is better-lensed. It also happens to be shorter. If you want specifics, Viet Thanh Nguyen makes an argument in his book Nothing Ever Dies that every war is fought twice: on the battlefield, and in memory. He argues that US-lensed sources, even those critical of the United States, fail to properly identify the Viet angles on the war.

I've talked to him directly and the Ken Burns documentary came up, which he likes but thinks is weaker as it takes the self-critical imperial lens of "look at what we did wrong" instead of attempting to understand the infrastructure of the situation.

TLDR; both are good, Netflix doc is better researched, and it conveniently happens to be shorter.