The romance novella is something that used to be a sort of ancillary product: generally, a way for publishers to bundle short pieces from popular authors with short pieces from new or less-popular authors. It was a mechanism as much for boosting those new or less-popular authors, as for promoting established authors.
There used to be regular collections of holiday-themed novellas; some Regency authors were so good at these that they would repeatedly appear in such collections. Now that the paperback collections are out of print, some authors are re-packaging the novellas. I have collections from Mary Balogh and Carla Kelly, and one from Barbara Metzger is on my wishlist.
In historical settings, holiday stories tend to lend themselves to novellas, I think, because it was acceptably realistic for an unrelated hero and heroine to wind up spending extended periods of time in each other's company. Historically, house parties would be pretty much the best opportunity for people to have repeated interactions - and, importantly, conversations. Physical attraction is one thing, but if you are going to have half a chance of really falling in love, you have to talk to each other.
In many historical settings, men and women were actively discouraged from monopolizing each other's conversation. The social standard could judge too much interaction as anything from bad manners to straight-up impropriety. And private conversation was completely off the table: an unmarried woman could not be alone with an unrelated man without considerable social risk. So the house party, which was ipso facto chaperoned but at the same time relatively informal, opened things up quite a bit.
Since e-publishing become such a thing, more and more romance authors are trying the novella format. Sometimes it's because they have a series of full-length novels but want to do a prequel, or a story that falls between novels. Sometimes - as in my case - it's because the piece is intentionally envisioned as a novella.
The challenge with romance novellas is that you want to convey a realistic love story in very few words. A novella usually clocks in at (average) 19,000 words; a full-length novel has at least 60,000. A full-length novel thus has a lot more room for character development, narrative description, conflict and resolution, and supporting characters. Its action also typically takes place over a longer period of time.
A novella collection I read recently had some serious issues. None of the stories were "bad," but out of seven authors, two were markedly better than the others. All eight stories revolved around an English country-house party encompassing Christmas and New Years. With eight love stories and eight sets of protagonists, the potential for wasted words was huge - and in fact that was the biggest problem.
Three of the eight stories (by two of the seven authors) I considered Very Good. Each of these three stories focused on its romantic leads and events key to their story, with just enough reference to the wraparound theme and just enough interaction with other characters. These stories also featured fewer anachronisms of language, better dialogue, and more realistic emotional interaction.
The remaining stories suffered from greater or lesser degrees of Too Much Other Stuff. Too much interaction with characters who were not the protagonists, or too much internal monologue (i.e. hero and/or heroine thinking about stuff rather than doing or saying stuff), and too much exposition generally. If you only have, say, 18,000 words to spend on a love story, at least 16,000 of those words had better be about that love story. Especially when the bulk of the love story is taking place in a limited period of time. Most of the stories had prologue in August or September of the story year, and wrapped up by New Years. That is not much time to resolve conflicts, and you can't be spending thousands of words reminding your readers that hey! this is a story set! and look at what all these other people are doing! when all you have is 18,000 words.
Consequently, most of the conflicts in the stories were trivial. Now, I am not a plot-dependent reader: for me, character and dialogue and setting are all more important. But I would happily read a full-length novel developed from any of the three stories I liked best, whereas I have no interest in a fleshed-out version of the other five stories. This is because the conflicts in those three stories were actually serious conflicts, the characters' approach to same was realistic and engaging, and the characters themselves were well-thought-out with back stories, hopes, and dreams that fit their historical milieu.
My goal with my own series of novellas is to have short, self-contained stories in which a single relationship is taken from inception to fruition. There is not a lot of conflict in my stories: it's usually just the conflict engendered when two adults try to combine lives. Sometimes in real life, this can get very bad - as in a case where two people fall in love, but one of them is already married with children. That is not the kind of conflict that can realistically be resolved within 20,000 words. Because my motivation for these stories is not to explore conflict resolution, I have avoided conflicts with that kind of gravity. My motivation is entertainment - for myself, and (I hope) for my reader.
Some of the L.A. Stories have relatively short timelines; others take place over a span of years. I don't believe an averagely competent reader is going to be prohibitively challenged by the concept of time elapsing offstage. The one I am about to embark on has its protagonists meeting and interacting for quite a long time before they become a couple. But it's still a novella, so all the intervening time has to be offstage. I mean, I could write the thing as a full-length novel; but there is actually a full-length novel I want to write that's related to the series, in fact intimately connected with the series, that's about other protagonists.
As I head into a series rewrite, I'm not planning to make most of the existing L.A. Stories significantly longer, or to introduce more conflict to make them "plottier;" I am, however, going to include a little more narrative, either descriptive or to flesh out internal monologue. I think of these stories as akin to a TV series with an ensemble cast in which each episode follows a few characters, while others pop in and out. They are, and were always intended to be, love stories - so the focus of each story has to be on the romantic leads. I just want to sketch in and color their world a little better.
The recurrence of supporting characters helps provide continuity. It also presents difficulties with continuity, but that's another story. Pro tip: start a timeline chart immediately. I've only committed a couple of continuity errors, but fixing them is going to be a pain in the tuchis. On the plus side, it's a lot easier to correct 2,000 words of a 20,000-word novella than 20,000 words of an 80,000-word novel.