Question

Why was is so difficult for U.S. troops to prevent supplies from reaching the viet cong

Answer 1

Because the supplies were being given from the Russians which had a underground network (both literally and figuratively) that transported weapons through out the country were the US could not attack.
Also, the Ho Chi Min trail was located in Cambodia which the US did not AT FIRST want to attack, and so it gave almost a clear safe path for both the Russians and NVA to get guns into South Vietnamese. Then from there, they would be given to the Viet Cong (who were underground). And then the Viet Cong would either use them, or give them to South Vietnamese people who were North Vietnamese or Communist sympathizers.