The main reason is accessibility to areas to hunt. Whether this is through no contacts or the expense of getting involved. There are no public hunting grounds here except the area on the foreshore that lies between the high and low tide marks. This can be arduous hunting involving rising in the winter hours before dawn ,traipsing out over the mud in the dark and holing up in a creek bottom in the snow or rain to pass shoot ducks or geese. All this with no guarantee of a shot ,with a 50 yard range over miles of foreshore that geese may flight over it can be a frustrating game.
Contrast that with say playing football with a team of kids your own age ,training during the week and competing at weekends . Playing golf where you can see improvement in your game . Participating in any of the martial arts ,Muay Thai, Karate, Judo Krav Maga, Boxing all freely available near here and all providing a satisfaction with progress.

My own son is a good clay and rifle shot and comes along with me but rarely asks when we are going. He is just not interested and instead has had a successful junior football career.
Shooting is not the be all and end all it is a hobby ,to some a way of life and freezer filler but it doesnt rule my life and Im not in control of its future Im merely a participant.

Politically I am not naive and there is an ever expanding anti fieldsports pressure in schools and television which will have a negative effect on the popularity of shooting.
On of the biggest negative impacts however is the association of sporting weapons with the American weapons free for all and the resulting carnage that hits the headlines. We are all tarred with the same brush so negative aspects of assault gun ownership are projected on to law abiding fieldsports men here .
I dont want uncontrolled access to assault weapons in this country theres no reason why I should just because I shoot clay pigeons or hunt deer doesnt mean I should support the unrestricted possession of military weapons for personal protection ,there is no link and its not hypocrisy ,its free choice. Should someone own an AR15 and thinks I should not hunt and so is against my possession of a shotgun that is fine by me its their choice.