This is probably not the best reason, but honestly I decided on romance because certain authors made it look "so easy". Prolific writers like Danielle Steele, and Debbie Macomber. I read their stuff, so incredibly popular and mainstream, and I started to think "I can do that."
Now when I would read the thriller/mystery/suspense genre (Lisa Gardner among my faves, who actually began in category romance) I would think: How do they come up with this stuff? I can't do that!
So I started with what was (cough, cough) "easiest" for me. And guess what? It hasn't been easy. But for me to dip my toe in suspense/mystery when my brain is already thinking "too hard" wasn't going to work for me. So I took the easy way out (still coughing here).
Now my family is after me to write a mystery. Why? I spend most of my TV time glued to true life crime shows like Dateline, 48 Hours, Investigators, and all the stuff I can find from the I.D. channel. I absolutely love it. It drives my family nuts. But I don't think I could write it!