My son has been watching and reading a lot about Anglicanism and The Church of Scotland numbers tanking, too. It's not just JWs that is failing to attract the young.
I suppose the major difference is the whole end times doctrine. The mainstream churches have at least the potential to recover and there are other ways into the church ie school, the Scouts, outreach homeless charities rehabs etc and of course traditional times we turn to the church within our culture - Christenings, weddings and services such as remembrance days (US veterans day) Christmas services, harvest festivals etc None of which Jehovah's witnesses have.
This is where they have a problem. They have no recovery point if the end times don't arrive which was their major selling point. They have none of these "inroads" for members that other churches have...like, at all.