Because they were beaten with overwhelming force.
Something...again...has not been allowed in the other places. Quite relevant.
Yes, beaten with overwhelming force, and then they actually surrendered and did not fight the occupation.
That's the entire question here.
Of course we could take over afghanistan. Of course we could take over vietnam. Somebody could conceivably pretend to establish an invasion of the United States.
But since I'm telling you this for about the dozenth time, I'll stop. Those things will not hold and the occupier or invader will lose, and will always lose, against a well-armed determined insurgency, especially if supply lines are long or difficult. Done.
I shan't explain it to you again. You are determined not to understand. Yes, we could've bombed the crap out of Vietnam and taken control over the whole place. And we'd still have lost and been forced home 5 or 10 years after that anyway, after you pissed away more money and lives.
Your occupation would fail. You can "win" the invasion, but you can't win the occupation. And that means you'll get sent home the loser no matter what.
We beat Afghanistan and Iraq with astonishing ease. And yet, we lost. That would only be 10x the case for anyone invading America.