This is a topic close to my heart and I can share some of my thoughts on why marriage can seem so intimidating.
There can be the fear of being so vulnerable with another person. Some people don't want to open themselves up to another person the way that marriage requires. There are plenty of reasons why, but this can be a huge factor. Past hurts, insecurity, lack of trust, or fear of failure. There are too many reasons.
I grew up with a lot of responsibilities and cared for my family, so the thought of marriage, which can be followed by thoughts of children, is tough. I know, it sounds terrible, but it's so much work. Single life can seem so simple and light in comparison.
Sometimes, it's easy to put off marriage because we see our faults or imperfections that we feel the need to try and fix before thinking about marriage.
One main reason I hear about is lack of certainty that marriage is right for them.