There are probably many reasons that could explain this development. The bottom line, however, is that our society has changed from a "Lord’s Day" mentality to a "Weekend" mentality. Allow me to explain what I mean by those terms.
In past decades, people worked during the week, but on the Lord’s Day they paused from work in order to worship God. In contemporary society, we work during the week and then feel that we need a weekend during which we can relax and be entertained. Our mentality has changed from Sunday as being a day to worship God to a mentality of a weekend focused upon ourselves and our desire to relax and be entertained.
The problem with the weekend mentality is that it tells us that, after a week of work, we need time to relax in order so that we can go back to work on Monday and work some more. (Ironically, I know some people who actually look forward to going back to work on Monday because they are exhausted from their weekend!) In effect, however, this mentality tells us that we are only created for work. In the old days, if a farmer had a mule, he would work the mule, but he would also give the mule some time off so that the mule would be ready to work some more. Our weekend mentality tells us that we are merely beasts of burden that need time off for rest and entertainment so that we can then work some more.
The Lord’s Day mentality tells us that we are not just beasts of burden. Instead, we have a God-given dignity. Jesus calls us together on the Lord’s Day so that He may tell us who He is and in doing so to tell us who we are. Jesus tells us who He is whenever we worship at Mass. He tells us that he is Abba’s Son. He tells us that he and the Father have a relationship so strong and deep that He and the Father are One. He tells us that their Spirit reaches out to us to invite us into this Communion, with God and our brothers and sisters, not only here and now, but for all eternity. This is why we exist, not merely to work, but to be a part of the communion of Father, Son, and Spirit.
God knows us better than we know ourselves. He gave us the Ten Commandments for our own good. We know that God meant it seriously when He told us not to steal, lie, kill, or commit adultery. But before God gave us these commandments, He gave us the commandment: "Keep Holy the Sabbath." He meant for us to keep this commandment just as seriously as He meant for us to keep the others.
God knew that if we forgot to worship Him, we would slowly but surely forget who God is and, therefore, who we are. This is what is happening in our modern society. As we increasingly look upon our weekend as a time for relaxation and entertainment, we increasingly drift away from the worship of God.