Because it is the closest port to mainland Britain with just 21 miles from the British port of Dover, Calais is one of the busiest cross-Channel ports on mainland Europe. Regular ferry services operate from Calais to Dover (P&O and Seafrance).
Hitchhikers can get over for free because drivers only pay for their vehicle and up to nine persons go for free with a car and two with a lorry – additional persons pay like footpassengers. If you are searching for a lift you often have to tell people about this possibility first – otherwise they often don't take you with them just because they think it's too complicated or they'll have to pay...