First, let's state the obvious: Most Americans live a fast-paced lifestyle. Many fill every moment of their day with activity. Unfortunately, it would seem that many people consequently have a limited attention span. To those people, change is exciting and stimulating. However, something that is predictable and unchanging is boring. They enjoy moving from one task to the next and then on to something else. This trend can be seen in many contexts (e.g., constantly flipping through the TV channels, changing jobs frequently, moving from one sexual partner to the next regularly, and even changing churches).
Part of the problem, as I see it, is a lack of contentment. People are looking for fulfillment and happiness, but they search for such in the wrong places. The apostle Paul declared - "I have learned in whatever state I am, to be content: I know how to be abased, and I know how to abound. Everywhere and in all things I have learned both to be full and to be hungry, both to abound and to suffer need" (Phil. 4:11,12). True contentment is found in fulfilling the purpose for which one was created. Humans were made to "fear God and keep His commandments" (Eccl. 12:13). No matter what else a person might be doing in life, if they aren't pursuing that fundamental purpose passionately, there will be a lack of contentment. When people feel discontented, they are likely to make changes.
Another factor to weigh is that our culture has gradually been embracing postmodernism. In other words, moral values are being increasingly viewed as subjective and flexible. In the minds of many, there is no absolute right or wrong. What matters to them is how they feel about a certain topic. Consequently, tolerance is the catch-word of our day and people today are not committed to principles of clearly-defined right and wrong as would have been the case with their grandparents. In the twenty-first century, the philosophy is: "I'm OK and you're OK, even if we don't agree on moral values or religious principles." People make decisions based upon what they desire as opposed to following absolute ethical principles or divine guidance from the Holy Bible.
Because religion and morality is subjective to so many, is it surprising that folks bounce around to various churches? Not really, since one church is as good as another in their mind. So, they pick the one that appeals to them the most (kind of like going through a buffet line with dozens of available choices). Tragically, most fail to understand that there is only one church (cf. Eph. 4:4; 1:22,23) and the way to eternal life is narrow and difficult (cf. Matt. 7:13,14).
What causes people to leave a church? There are various reasons, but typically they are dissatisfied. Perhaps they disagree with certain teachings being proclaimed. Perhaps there are personality problems or conflicts they are unable to work through. Maybe they feel the congregation is too small or too large or that it doesn't have enough activities, etc. Some people just feel they'd be better off somewhere else, so they leave and are warmly accepted elsewhere.
In the congregation for which I preach, we've have had our share of religious churn over the years for many of the above reasons. Numerically, we're about the same as we were seven years ago. However, the composition of the congregation is completely different. We've lost a lot of members to death, moving away, and apostasy. We've had some children born, some new converts added, and some folks move in. Sadly, we've had a number of people who obey the gospel of Christ and then leave when they are faced with challenges (cf. Matt. 13:20,21).
What can be done to stop the churning? Well, it may be tempting for a congregation to compromise their convictions to attract people. But, this is certainly not the answer. A wise man once stated: "If you hamburger people into the church, you'll have to hamburger them to stay." We need to attract people to the gospel message for it is God's power to save (not programs and activities; cf. Rom. 1:16)! True and undefiled religion is not about getting people in the door. It's about molding people via the power of God's word to be like Jesus (cf. I Cor. 11:1). It's about helping one another live pure and benevolent lives (cf. James 1:27). In order for that to happen, people have to learn the truth of God's word and be fully committed to it! This will lead to maturity and stability on an individual and congregational level. That is the only sure-fire way I know of to stop people from leaving the Lord's church.