As a churchgoer (intermittently at present), I think it's down to three things:
- More people being atheist/agnostic/other religions than has been the case in recent history in the West;
- A growing sense of not needing to go to church to affirm one's religiosity or adherence to belief;
- Churches not always being very inclusive or welcoming, and instead becoming cliquey, judgmental and downright hostile to people who don't conform to their ideals (despite their religion often encouraging the opposite kind of behaviour...)
That said, attendance levels do tend to go through peaks and troughs - my local church has started to get new members recently, as have others in the area. At the same time, other churches have had to close over the last few years due to falling numbers. It all depends on how appealing a church makes itself, and how relevant people feel it is to their lives.