In World War II especially in Europe the Nazi's wore a very distinct uniform, they had no guerilla fighters, booby traps (although there were some) were very rare, the American people were still furious about the bombing of Pearl Harbor and they fully backed the war to the point where car factories became tank factories and house wives did anything they could to help the war. Propaganda was everywhere and also troops got support from the country. Also Europe is a very open environment, more tank warfare was utilized and you could see a German from a mile away. And lastly, we had most of the world helping us take down two European nations. USA, UK, and Russia alone did most of the work. However Japan was where we had the most trouble. The dense forrest was utilized by the Japanese and they were a fearless people who would literally surrender and then pull out a grenade and kill 4 or 5 people. That place was so miserable moral was extremely low also.
Now in Vietnam even though we were fighting the North Vietnamese Army(NVA), in the South people known as the Vietcong were dressed as civilians and Americans didn't know they were bad until they shot and at times killed someone. Booby traps were scattered around all over Vietnam. The forrest was so dense and NVA would set up bunkers in the thick forrest just waiting for an American patrol to go by and they would light it up. Also, this war was not backed at all by the American people. Most people wondered "Why the Hell are we even there." When soldiers came back from Vietnam people would spit on them and call them baby killers. Protesters sometimes stood outside the White-house and set themselves on fire. The South Vietnamese Army was so bad as soon as we left Vietnam the North took over. And lastly we barely had any help in this war unlike WWII.
Notice how in a thick jungle environment wars (Japan+Vietnam) a mainly conventional force such as the US has a much more difficult time with unconventional forces like them. If we had full approval of the people and stayed in Vietnam we would have won that war.
Unconventional warfare ***** up conventional especially in an environment best for it.