In charge of is a complex preposition like in place of, in spite of, in view of, or in case of. It means having responsibility for or jurisdiction over someone or something. In the charge of is a preposition phrase meaning in the care or custody of someone.
A similar change in meaning occurs when the definite article is inserted into the complex preposition in view of. In view of means because of and in the view of means in the opinion of.
And She was sitting in front of the car (complex preposition) has a different meaning from She was sitting in the front of the car (preposition phrase).