It isn't a question of being single = happy, being coupled up = misery. You can be single and desperately lonely, and you can be married and blissfully happy. The point, I think, is that people (not just women) have woken up to the fact that you don't have to form a relationship. Until very recently, women had to have a man. If you were single/unmarried, you were vulnerable. You couldn't own property, couldn't vote, and couldn't access higher education. If you were young and poor, and you didn't have a husband, people assumed you were a prostitute. You were also likely to be sexually assaulted/raped (and good luck complaining to the police!).
Even when I was a kid, in the late '80s, it was considered odd never to marry. And as a teen in the '90s single women were still thought of as failures. At last things have begun to change. We're not there yet, but society is beginning to see relationships (and kids) as just one option among many rather than 'the norm'.
Also, we're more aware how different people really are. Some are introverts, some extroverts. Some are highly sexed/sexual, and some aren't. Some crave and need intimacy, and some are happy on their own. It isn't so much that being single is great, more that it's great for quite a lot of people. In the past, women grabbed the first half decent man they could find, had kids with him, and then made the best of things. Both my sets of grandparents had awful marriages – awful. I remember my grandmother saying "this isn't normal" (meaning being stuck with the same man for 60 years). So many women endured utterly hellish marriages with oafish, abusive men because even that was better than being pitied and shamed by society. Even the women who found decent men often hated their life because they just weren't suited to marriage.
Personally, I'd say that many people aren't suited to long-term relationships. It isn't so much that they've never met 'the one' but that they're just not cut out for it. They're too selfish, or independent, or introverted, or traumatised, or restless, or highly sexed, or whatever it may be. And even the people who are suited to marriage often end up with the wrong person. So many marriages implode (my cousin was married for 30 years but his marriage has just collapsed, and his brother has been married twice, both times a disaster – lovely, lovely guys both of them, but it just never worked). And even the ones that don't implode are often miserable. So yeah, not so much that people love being single, more that they're aware how hard, and painful, relationships can be, and would rather fill their life with books and art and dogs and nature and friends. Of course, the big problem is sex. I genuinely think it causes us more misery than happiness. It wouldn't be a bad thing if, at some point in the future, we could find a way of literally switching off the sex drive (if we wished). God it would save so much pain.