It's hard for me to explain. Basically it has to do with intent. If a couple moves in together to test the waters, they know that they can always bail out if they don't like it. They are not promising to be together for the rest of their lives. They aren't being serious about it. If they start off with a casual attitude toward the relationship, they are likely to keep that attitude after they get married. A casual attitude toward marriage is not ideal if you want the marriage to last forever.
There are many people who live together and then go on to have great marriages. My wife and I are one such couple. But statistics say it's rare. A quick Google search turned up this
marriage counselor's site, which says that 85% of all couples that lived together before marriage ended in divorce. That statistic may no longer be true, but I bet the number is still high. Do a Google search on it. There are a lot of religious sites you have to wade through, but there's some info out there.