1) Selfishness. People are selfish, and that is fallen human nature at work. You can't blame a generation of poor catechesis on this. Selfishness is overcome by the constant practice of giving yourself away, which is an "action", not something you read about. Actions of love are the responsibility of parental direction. Kids who have not been challenged or had a proper training of their will just don't have the tools as an adult to give themselves away. Therefore, they are not yet ready for marriage, and they spend their 20's figuring that out.
2) Fear. 20-somethings today are afraid of so many things. Can I afford to be married? Have I done enough yet before I settle down? Will a marriage I am in end in divorce? Is the world overpopulated? Is society too evil? Is the world going to end? I am exaggerating here, but the point is that fear keeps young adults from ACTING in courage on their vocation.
3) Ignorance. 20-somethings today are actually highly intelligent. And we are seeing more and more of an increase in love of the Catholic faith in young adults. But they are very ignorant when it comes to their vocation and what it means to get into it. And even those who have strong faith believe it is a good enough excuse to be "busy" in works for God (i.e., missions, serving the poor, teaching, etc.). They certainly don't understand that the prime TIME to get married is when you are in your 20's, or that it is healthier for women to start having children in their 20's, etc. But most important, they don't realize that God wants them in their vocation sooner, rather than later. And He can make this happen if the person is open. Too much focus is on the "who" in marriage, instead of the "when".
4) Too Many Options. I believe single Catholics have a strong awareness that there are many people out there who are "available", and there could be a tendency of putting off being open to a perfectly good person due to a hope that there might be somebody better out there, or have the idea that maybe this is not the one "God has chosen". God does assist in putting people in our path, but He certainly does NOT have one person chosen from all eternity for you. That is a choice that is left to the individuals. God simply blesses the choice, and then at that moment of making vows, that is the one person for you.
I realize people will say that if you don't meet the right person then you may not marry in your 20's. Well, that's another discussion and set of theories to explore. Suffice it to say that there are enough Catholic singles in their 20's that are not getting married simply because they choose not to. Life is too short, our vocation is too important, and having children and giving your life to others for a lifetime is critical to getting to heaven. Those who know they are to be married would best serve themselves (and God) by focusing on a time-frame to work with God, and then choose a good person who will make a good spouse and parent, who loves God and seeks His Will, and who knows how to forgive and ask forgiveness. Everything else can be dealt with.
NO ONE GETS OFF FREE OF THE CROSS IN MARRIAGE. No one! So you might as well be in your vocation where the grace of the sacrament is available to perfect you on your path to heaven.