Why did Diplodocus had a very long neck?

Diplodocus had a very long neck primarily to help it reach high vegetation and access food that was otherwise out of reach for many other herbivores. This adaptation allowed it to feed on leaves from tall trees, giving it a competitive advantage in its environment. Additionally, the long neck may have assisted in foraging over a broader area, allowing it to consume a larger quantity of food with less movement. This unique feature also likely played a role in social interactions and mating displays.