It is because these are the departments that generally deal with people and ensure a two way flow of information between the customers and the company. The back end offices will include departments like finance or operations that deal with the production or manufacturing of products and the running of the company it is the marketing or sales department that get in touch with the people and lets the company know what sort of expectations is out there in the minds of the customers. The service departments like customer care also constitute the front office of a company.
Example: for example, in the hotel industry the house keeping department and the reception desk constitute of the front office. In a typical FMCG company the marketing and sales team constitute the front office.