(Sorry about the ads. Are they relevant?)
Before you read my thoughts about the question, I suggest that you think about your own answer. I even suggest that you record your own ideas in a draft email, because that is what you think. If you write afterwards, then you will be writing partly in reaction to what I wrote, but I am especially interested in what you think, not what you think about what I think.
As a preface, I want to comment about how difficult communication really is. We can't read other people's minds, but we have to try to understand what other people are thinking. I think of real communication as the sharing of mental models. We use words to describe pieces of our own mental models. If two people succeed in communicating, then the second person will create a mental model that is similar to what the first person was thinking about. It seems trivial because we do it very often, but when you think about it more deeply it isn't such a trivil thing. To really communicate effectively, it's not enough to understand what you think. You also need to have some mental model about the other person's mental models... That's why I recommend you make some notes about your own thoughts about the question. Your own mental models have to be the starting point of any real communication.
One more discursion. It's kind of a warning, actually. Sometimes I even upset people with my writing. I think that's mostly because I write very firmly and clearly, even about difficult topics. It's actually my job as a professional editor to help other people write very clearly, and I believe I'm pretty good at it. However, the style sometimes creates the impression that I think I have absolute answers where I'm only describing speculations or ideas.
Reasons why people don't marry (or are unhappy with their marriages):
Boring reality versus unrealistic
On television, in books, but especially in movies, the relationships, and especially the romantic relationships are highly idealized. People accept these models as personal goals, and then are disappointed by real people who are just being natural.
Freedom versus compromise
Marriage involves lots of compromises and giving up certain freedoms. For many people, but perhaps especially for Americans, being free may seem better than having to compromise with someone else on many things.
Work and economic constraints
Many people are just very busy with their jobs and don't find time for social activities that might lead to marriage. (Or their work even causes them to neglect their families.) An especially common form of this problem is to give priority to 'building a career', especially when people have their first jobs.
Poor interpersonal skills
Not clear to me if such skills are mostly natural or mostly acquired, but it is clear they vary widely. As noted on my 'greeting card', I'm basically uncomfortable at parties and with meeting strangers. Other people love crowds of strangers.
That's actually just a preliminary outline of the topic. The answer of my own life is rather more complicated, though it includes elements of all three. I think #1 is less relevant to me than when I was younger.
If you wrote your own ideas before reading mine, then I'm very curious what they are. Also, I'm curious about your reaction to my thoughts. Perhaps I'm just a curious person?