May i offer a couple of reasons offhand as to why people backslide -
1) Trying to make God in our image rather than letting God be God and allowing Him to be Lord as well as Saviour in our lives. Many people will say, "God did not do so-and-so for me so He either does not exist or, if He does, is not worthy of my devotion because He didn't answer my prayers." God as magical genie - an insurance policy to keep us "healthy, wealthy and wise" and as a 'get out of hell free' card - does not cut it. He is the Lord - we must continue to have faith in Him even when things don't go our way or it seems as if He is not answering our prayers (in a way which we would like Him to)
2) 'Throwing the 'baby' out with the bath water' - judging Christ and Christianity by the actions of His/it's followers. Christ and His word are infallible, His followers are not. Some people are not discerning enough to realize this.
Ray