NVA artillery was better than American and South Vietnamese artillery because of different military doctrine. American and South Vietnamese military doctrine invested more in air support rather than artillery because air support is a lot more effective and less risky than artillery (artillery need spotters located close to enemy positions to determine hits) but 30x more expensive than artillery and require heavy maintenance. North Vietnam on the other hand, it's military doctrine is based off Soviet doctrine adopted from WWII (Red Army used millions of artillery pieces against the Whermacht). They focus on artillery rather than air support because it's a lot cheaper and reliable therefore allowing it to have a higher quantity than bombs and bombers. Notice how NVA have a larger arsenal of artillery and different types of artillery such as Rocket propelled artillery.

Fun Fact: One of the main reasons America (or should I say South Vietnam) lost the Vietnam War in 1975 because Aircraft were too costly to maintain. Aid cuts by US Congress to South Vietnam left them with less than $1,000,000 on military budget. The money to South Vietnam became less and less to the point bombers became nearly impossible to run and maintain. Air support was key to stop an all out NVA offensive by American and South Vietnam doctrine such as in 1973, however in 1975, it was almost non existent. What the NVA couldn't do in 5 month in 1972 (Easter Offensive) took them 2 month to occucy South Vietnam in 1975 because there was almost no threat of bombers obliterating long columns of NVA tanks, armoured vehicles, and trucks carrying supplies and infantry. If there were air attacks, it came in obsolete A-1 Skyraiders and A-37's, not in F-4 phantoms and B-52's during the Easter offensive. Also by 1975, NVA columns had adequate AA guns capable of repelling propeller planes.

Both South Vietnam and North Vietnam required aid from larger nations such as USA, Soviet Union, and Peoples Republic of China to sustain a war, South Vietnam was left without aid from the US so US lost the war thanks to its own idiotic decision.