In the novel, Buddha does not give Zhu Bajie the title because he asserts that Bajie has not yet attained victory over his weaknesses.
You guarded the holy monk on his journey, but your heart is still unregenerate, and you are not yet purged of your lust. But as you won merit by carrying the luggage, you will be rewarded with promotion as the Altar Cleanser.
Bajie is so angry at hearing he will be denied the title of Buddha that he immediately demands to be told why he has been denied the privilege. After all, Sun Wukong has just been designated the title of Victorious Fighting Buddha, while Xuan Zang has been accorded the title of Candana-punya Buddha.
Tathagata (the title which Siddhartha Gautama calls himself) responds that Bajie is still lazy, gluttonous, selfish, and lustful. He maintains that Bajie should be pleased to be given the position of Altar Cleanser, as his job involves cleaning up after all Buddhist services; because food is often offered as a sacrifice at altars, Bajie will have plenty to eat as a result of his position.
In The Journey to the West, Zhu Bajie was originally Marshall Tian Peng, the once immortal Water God of the Heavenly River. He guarded the gates to the Jade Emperor's Palace with the Supremely Precious Gold−imbued Rake, a magical and powerful weapon. However, in his drunken state at the Peach Banquet one day, Bajie used his powerful rake to force the moon goddess, Chang'e, to respond to his lustful will. For this sin, Bajie was banished to be reborn on earth. However, on the way to earth, his spirit somehow ended up in the womb of a pig, and Bajie was born with the unfortunate, ugly face of a boar.
Bajie was given the new Buddhist name Zhu Wuneng (Pig Awakened to Power) by Guanyin, the Goddess of Mercy, after he agreed to protect Xuanzang on his journey West. Despite all his exploits on this journey, Bajie never took effective control of his appetites and thus forfeited the title of Buddha when rewards were conferred on the four travelers.