This is because he has a down-to-earth but not brave enough person who can't stop and persuade his son's words and deeds, which eventually leads him astray.
From several stories revealed in the article from Darcy's mouth, we can know that in the eyes of old Darcy, Wickham's father has always been a respected housekeeper, while his mother has squandered all the money in the family, so there is no way to provide him with a good education.
Old Wickham can't control his wife's behavior, nor can he set a correct example for his children. Such a family order is doomed to be chaotic and lacks unified family behavior and values. Old Darcy gave Wickham selfless care and support, sent him to Cambridge to plan his own career, but did not let him get corresponding control, but let the dark side of the outside world turn him into an idle and full of nonsense.
Wycombe has a profligate, lazy and irresponsible mother and an honest and practical father. But instead of inheriting his father's advantages, he learned his mother's shortcomings. In addition, he learned to disguise, and honest old Wickham may not be able to control his children.
In addition, the love and care of old Darcy did not make him better, but encouraged Wickham's greed. At this time, his father did not reveal his behavior to old Mr. Darcy, so his default indulgence made him out of control.
To this end, he pretended to be decent in front of old Darcy, and in order to get money, he pretended to be a gentleman and a victim to defraud trust. He knew that being a priest and a soldier could not make money, so he chose to seduce rich girls and gamble.
Under the influence of his mother's doting, his father's neglect of teaching and the negative influence of society, Wycombe grew up to be the most morally corrupt person in the world. For him, his biological father's education is laissez-faire, and the godfather's education is non-instructive, which leads to his character defects and many problem behaviors.
But family education is on the one hand, and Wickham's nature is the main reason why he became a moral corruptor. It can only be said that the angry Darcy knew Wickham's true face from the beginning.