Going door to door is an obligation in the JW's. Giving a bible study isn't. You can ask people if they want to study the bible with you, but few are those who will accept.
There is lots of work and research to be made in order to answer all kinds of questions when a person is interested in a bible study. Did this responsibility frighten you a bit?
Were you just satisfied in going door to door and accumulating your hours in the field service rather than really wanting to start a bible study with someone?