Mass media has replaced religious institutions as sources of information and authority in people's lives
Religion now competes with a whole range of other ideas in the spiritual supermarket
Scientific explanations (starting from the enlightenment period) have replaced faith in religious ideas
However, some people will return to religion when science and technology has no control, e.g. incurable illness
Some churches are too traditional with their stances on divorce, use of contraception, abortion, sex outside marriage, illegitimacy and homosexuality
Fragmentation of belief - there is a diversity of different religious faiths and organisations, and traditional religions have to compete with tarot cards, paganism, spiritual healing, astrology, witchcraft and beliefs in the paranormal etc.
For many churches, particularly Christian, the main service is on a Sunday and for lots of people this will not be a good time (whether it be taking children to football, cooking a sunday roast, or just generally having a rest after a hard weeks work)
One of the main arguments is that science is a rational thought based on evidence and therefore it explains everything. However, it doesn't always. For example, in 2008 a NYC window cleaner survived a 47-storey fall. Doctors (Medicine/Science) would say that he should have been dead .. so why isn't he?