There is no general answer to this because every individual is different.
But, I speak out of personal experience and what according to me seems like the reason for many people to be single.
I am 28 have been single for life. Have dated just one guy and that relationship too didn't last due to a lot of reasons.
People who have stayed single for too long or prefer to stay single can do so for the following few reasons:
1.The partner you chose is not ready to some terms and neither are you ready to compromise on what you want for yourself.
2.A sense of freedom and the need of not being answerable to anyone is something that is worth a treasure.
3.The number of responsibilities when you are single are way lesser than those that are on people who are married or in relationships.
4.When you time and again see the person you're in a relationship with, calculative about every penny you're spending together, without the need to inherently do something for each other. You give up on it.
5.When you have too many other priorities which potential partners cannot understand or adjust too… You'd rather live in peace than squabble over it everyday.
6.When you've been through a heartbreak and someone who told you you were irreplaceable replaces you in a very short span of time, trust and love feel like fiction and lies. And then you want to save yourself from going through the same struggle again and again.
These are the probable reasons why people stay single in life. Less drama, more freedom.