The Anglican Church has had its share of internal fights and arguments.  These often drive people away.  I don't really know enough about other mainline churches to be able to talk about the reasons for their declines.
I do think that it would be worthwhile for the Catholic Church to look at the reason of the loss in membership.  I know that many people talk about small home-churches being the future.  But what happens when one does not know people who are interested in that?  I think that organized religion can serve an important role in society, but it seems that those in charge don't really want to do so.  I think that's a shame.