But why is that? I honestly can't understand it
Well, you have only so much money, and you get the Army, the Air Force and the Navy all trying to get their piece of the pie. But the Marines were part of the Navy, and the Navy didn't like them, so when they got their piece, they only gave a few crumbs out to the Marines.
I don't know. There was big conflict between Gen Walt and Westmoreland on this. The Army fought from a defensive position and Marines from a offensive position. A good example is the battle of Dial Do. At Khe Sanh the Marines fought mostly in the surrounding hills. Sort of the job of the Marines was to take the fight to the enemy rather than getting them to bring the fight to khe Sahn.