But if that's the only kind of book offered to readers each month, that's all we have to select from when we make our purchases. There is a commonality if the focus is wide enough. Once we find an author we like, we stick with her. Several times I've seen a reader post on TRJ's bulletin boards, and I've even said it myself, that we'll read a grocery list that Author A has written. Personal preferences in terms of favorite plots, character types, setting, etc.
Some readers likes their stories to sizzle with sex.
I'm an easy mark for an unrequited love story. I doubt there is a veteran romance reader out there who hasn't got a mental cheat sheet of auto-buy plots. We all want to be entertained by our reading material. For some readers, that is the escapism aspect of fiction. Others want to sink into the time period and learn something.