Short answer – because I’ve always written romance!
Well, not always. My first stories, as a pre-teen, were school, pony or theatre stories, but once I hit adolescence, I read romance, I watched romantic films and I wrote romance. I wrote stories for my friends which had 1940’s style romance between our favourite teachers or pop-stars or even the boys we liked. I’m sure I could be sued if those had ever hit the light of day! Of course, even though those stories were based on ‘real’ people, they were still imaginary really!
My first published novel in the 60's was a re-hash (and fuller development) of a story I’d written in my teens – names and places changed, of course. In later novels I developed my own main characters, who became as real to me as the earlier characters in my stories. I’m a romantic at heart, maybe because of (or despite) the fact that real life has proved different for me.
In a sense, I came back to novel-writing by the same route - writing fan-fiction stories about people I'd seen on the screen in The West Wing.
There’s no way I could write detective, mystery, suspense etc stories – my mind just doesn’t work like that. And I am simply not interested in writing (or reading!) what seems to be a modern trend (in America anyway, though I think less so in the UK) for paranormal, fantasy, vampire etc etc. I like my characters to be real people, not fantasy characters. One day I might try a historical – but it will still be a romance.
I think many women need some kind of escapism into a world where the heroes are gorgeous and romantic, and say and do all the things they’d love real-life men to do and say (but often – usually? - don’t). Otherwise why would romantic fiction be so popular?
And anyway, I like falling in love with my heroes.