So here's a couple of things:
Mainline churches - Lutheran, Episcopal, UCC, United Methodist, etc. - have been declining for years now. Conservative churches - SBC, Assemblies of God, various independent congregations - have been growing for years now.
Clear so far? Good.
Ok, Conservative Evangelical churches require people to have a particular type of experience and lifestyle in order to belong. They generally do not allow smoking or drinking, and will often frown on dancing. Being a Christian requires being "born again," which involves a specific confession of one's sins and need for forgiveness. These churches often will not baptize infants at all, and will only baptize children if they are judged to have attained sufficient age for understanding what it is they are doing. Baptism, then, is not initiation so much as it is testimony, a public statement about one's new status as a born-again Christian. These churches will also usually re-baptize anyone who was baptized as an infant in another tradition.
This last is particularly important to the point I wish to make. Within the Conservative Evangelical movement, estimated at around
20 million adults in the USA, Mainline denominations are usually characterized as being made up of a few "real" Christians and a lot of "nominal" Christians - people who say they believe in Jesus and the Bible but who don't actually have a "personal relationship" with God.
Here's the point of all this: Conservative Evangelical churches are growing by gaining the people who are leaving the Mainline denominations. They are growing by attracting people who wish to redefine their Christianity, rather than those who actually have no previous connection to the faith.
Obviously, this is not true 100% of the time. But the Church - the whole deal, capital-C Church - is declining overall. The number of attendees is going down, as are the numbers of people who say they believe in God, those who claim Christianity as their religion, whatever that may mean, all across the board, the numbers are going down.
And the only "growth" to which Christians can point is cannibalism.
This may also partially explain the phenomenon that Sanpete observed in comments about there being a wider divide between Liberal and Conservative Christians than between Christians and non-believers. I don't totally agree with him, and I want to deal with this in depth a bit later. But Liberal Christians have spent years being demonized by the Conservatives, and have seen their members leave in order to go to these newer, more tchnologically advanced and more "certain" churches. They can't be happy about it.