Because a wedding is actually a SERVICE the Church offer, like selling something, whereas, the Christening is a ceremony to bring members into their fold, which a wedding is not,.
Historically, a wedding took place in the Brides Parish, and often this meant a groom was from another parish, and that meant that members of one Parish, who paid Curch Rates in that parish, were then crossing Parish Boundaries and using the services of another Parish, for which they hadn't paid in any coffers, therefore someone had to pay, and as the Bride's family had sanctioned the marriage, they had to foot the bill.
Over the years this has now excalated and the cost is more for the Wedding Breakfast, but the Church fees have to cover the ceremony taking place outside of the set services the Church hold, plus, weddings were not allowed on a Sunday anyway, and so the Church had to provide the facilities as a special opening time.