I think one of the biggest reasons for the decline of marriage is feminism. This evil has destroyed the home, family, society, and frankly, the whole world. As women go, so goes society. Men do whatever is necessary to have frequent and convenient access to sex. If women insist on marriage, men marry. Women hold the power of setting societal standards.
Also, the sacrament of HOLY MATRIMONY is not properly honored anymore. In the secular world, divorce is an easy way out of a life -time commitment. When you marry, it is supposed to be UNTIL DEATH do you apart. End of story. You better make it work. Aside sentimentality, the first and foremost purpose of marriage is the upbringing of children, soldiers of Christ, and the propagation of the human race. There is too much focus on "love", or the ideal marriage, which do not exist. Sentimentality that just gives people false expectations, which lead to great disappointments.
Truth is, marriage has lost its spiritual meaning. People do not wish to commit anymore to anything nor anyone. Another symptom of this society we live in, which is dominated by narcissism, individualism, consumerism, materialism, and moral relativism.