There are probably several reasons, but I think one that is often overlooked is the lack of a feeling of ownership. I think it is critical that each member feel needed, wanted and appreciated.
Having the support of experienced adult mentors is, of course, important and welcomed. However, while the adults should be there to provide support and guidance, it is equally important that the young people feel that they have ownership of the club. If they feel that they are a valued member of the group, they are more likely to be active, long-term participants.
That feeling of ownership can be conveyed in several ways. You need to take every opportunity to give that feeling to each and every person.
You are bound to have a wonderful mix of talented people in your group: artists, writers, photographers, social networkers, and more. Allow the members to use those talents. Do you want a logo? Invite all of the members to submit a design and have the membership vote on their favorite. Do you want a newsletter? Ask the members to be reporters and write articles. Need photos of the club's adventures? Recruit members to be photographers. How about a social network? Ask members to be the administrators. Do you want to host an event to inform the public about your group's focus? Invite the members to create posters and displays, similar to those used in a science fair or 4-H.
Your club should be making a statement that it is an organization by young people, for young people. It is a place that needs the participation of each and every young member. It is a place in which the young people can feel empowered and appreciated for their unique talents. It is not an exclusive organization run by a select few; it is inclusive and willing and anxious to utilize whatever skills each young person can bring to the table.
If you want your group to be successful, make sure every person is active in some meaningful way. Be sure to recognize each of their contributions and let them know how much they are appreciated. The success of the group, and the success of our conservation efforts, depend on the efforts of all of us!