Why didn't Niu Wangmo catch in The Journey to the West, but the Monkey King?

Chapter 60 the Monkey King Pig Bajie and the sand monk beat him, but later the sand monk guarding the Flame Mountain could not beat him. It is a little difficult for the four of them to beat an old cow. Finally, the old cows showed their noumenon and had no idea at all. Immortal Buddhists don't send troops, and old cows help protect the law. It's too easy to take the old cow to the Western Heaven to learn the scriptures.