The fact is that the US military outfought both the Viet Cong and the NVA in most battles. The famed Tet Offensive wiped out the VC, and it wasn't until the US had basically cut off all military support in 1975 that the NVA was able to successfully invade the South.
This is what you would expect when you compare the size and scope of each countries military. It doesn't denigrate anything the Vietnamese people accomplished in their fight for a unified, independent country.
The Vietnamese were a hardy, tough opponent, that demonstrated incredible creativity and ingenuity in fighting a much wealthier foe. In small unit battles, they were as good as any US troops.
US troops also fought well, contrary to the stereotype of drug addled draftees who fragged their officers while committing war crimes.
Acknowledging the things that benefitted each side isn't trying to diminish the skills they exhibited. It's simply acknowledging the battlefield they fought on, and all of its constraints. The VPAF had both constraints and advantages as they fought a much better equipped foe.
If I had to risk an opinion, I think it's because it's a memory of a painful defeat for the US -- a militaristic nation with pride in its war history -- and so explanations are devised to diminish the skill of the Vietnamese and to explain away the defeat due to external reasons ("the politicians abandoned us", "the public didn't support us", "the Vietnamese were treacherous and didn't fight fair", "our leaders didn't let us use every weapon at our disposal", "the Vietnamese had help from China and the USSR").
That the Vietnamese fought genuinely better than the US isn't an acceptable explanation, which is why it's remarkable that upon study of the war, this is the fact that emerges.