It is a truth almost universally acknowledged that the heroine of a romance novel must be in want of a mother. She may be an orphan, or she may be the abused/neglected child of a toxic mother, but the vast majority of romance heroines are somehow separated from a mother's love.
There are compelling reasons for this. Isolating the heroine from unconditional love makes her eventual connection with the hero that much more important. We feel sympathy for her, which makes us more inclined to read her story and cheer for her eventual Happily Ever After. It is slightly less common for the hero to suffer a lack of mothering. He is a bit more likely than the heroine to have a toxic mother (it is often the reason for an alpha hero's general mistrust of women, bordering on misogyny).
I find myself much more willing to read a story about an orphan heroine (or hero) than one with a toxic mother. That might be due to the little bit I've read about human psychology. An individual who has been completely robbed of a mother's love is not well-equipped to accept love from others.
I remember reading about the heartbreaking experiments that Harry Harlow did with rhesus monkeys at my alma mater (I took a psychology course from one of his former teaching assistants). Baby monkeys were separated from their mothers. Some were given occasional physical contact with their mothers, and some were given no contact at all (a control group remained with their mothers all of the time). The ones who received limited contact were very clingy and anxious. In a group, they all huddled together. The ones who received no contact were hostile and aggressive. They did not like to be touched and would respond to overtures with violence.
Our parents are the only people from whom we can reasonably expect unconditional love. When their love is deliberately withheld, it damages our ability to trust and connect with others. I find it difficult to suspend my disbelief when a heroine raised by a truly toxic mother is transformed almost overnight by the love of a good man. It mirrors the notion that a hardened criminal can be reformed by the love of a good woman. That may be a popular trope in motorcycle club romances, but in real life the best that such a woman can look forward to is occasional conjugal visits for good behavior.
Looking through the historical romances I have enjoyed enough to re-read, most of them feature heroines who were orphaned but knew the love of parents or guardians for at least part of their formative years. Some are those rare romance novels with heroines whose mothers are still living and not toxic (although they may be somewhat challenging).
Julia Quinn's Bridgerton series - Although my favorite Bridgerton novel is actually about one of the sons, there are four books featuring heroines raised by loving family matriarch Violet Bridgerton. Of those, my favorite is To Sir Philip, With Love. (Perhaps not) ironically, Hyacinth becomes a loving stepmother to children whose own mother neglected them.
Confessions from an Arranged Marriage by Miranda Neville - Minerva Montrose's parents are both living. They are eccentric but loving, and their children are all well-adjusted. Minerva's sister Diana is the heroine of The Dangerous Viscount, so that is another book I could name with a living, loving mother.
The Countess Conspiracy by Courtney Milan - I have to give a special Mothers Day shout-out to this book. Violet Waterfield has a formidable mother. She has imposed rigid rules on her daughters to help the family weather the old scandal of their father's suicide. Violet has always feared her mother will reject her if Violet reveals her shocking scientific studies. In the end, her mother's love is truly unconditional and unlimited. Violet must learn that before she can accept Sebastian's love as well.
A Gentleman Undone by Cecilia Grant - Lydia Slaughter's parents stood by her when she fell pregnant out of wedlock. They uprooted the family and moved to a village where no one knew her, to help shield her from the consequences. After their tragic death (I don't recall the circumstances, but I think it was a carriage accident), she blamed herself and embarked on a self-destructive path. Will Blackshear eventually convinced her that she is worthy of love, but that would not have been possible if she had not already known the love of her parents.
Secrets of a Scandalous Heiress by Theresa Romain - Augusta Meredith is mourning the recent death of her parents (in a boating accident rather than a carriage accident) and the defection of her lover. She longs for human connection to comfort her in her grief.
The Spymaster's Lady by Joanna Bourne - Annique Villiers is mourning the recent loss of her mother (whose carriage went over a cliff). Madame Lucille raised her daughter to be an elite spy for the French Secret Police, and Annique has been an exemplary pupil. Over the course of the book, she learns secrets about her beloved mother that force her to question everything she believes. In the end, though, she forgives her mother for the choices she made and accepts that she truly loved her.
A Week to Be Wicked by Tessa Dare - Minerva Highwood's mother is clueless and social-climbing, but she does love her daughters. She reminds me very much of Mrs. Bennett in Pride and Prejudice. Minerva is shy and socially awkward, but she is not emotionally damaged. She is well able to fall in love and be loved in return. Minerva's sister (another Diana) got her own novella, Beauty and the Blacksmith.