This is not true of fiction in general. Scrooge needs to change. Crachit, not so much.
On the other hand, Javert does need to change, precisely because his entire arc is tied to the main character, Jean Valjean. Although Jean Valjean's change occurs many years before Javert faces the pointlessness of his relentless pursuit, the one change in perception is contingent on the other.Likewise, in romance, one change incorporates the other. One character may change more--Emma definitely changes more than Mr. Knightly. But Mr. Knightly also undergoes an epiphany. In Persuasion, uniquely enough, the primary protagonist, Anne, doesn't change as much as the lover, Captain Wentworth, but she does change. In the final discussion, she has come to terms with her past and has the confidence to assess her decisions objectively. She has, in a sense, forgiven herself.
Romantic fictions in which only one partner changes have an oddly unbalanced feel. Sense & Sensibility, Austen's first book, has such a feel. But then, arguably, the partners here are the sisters, Eleanor and Marianne, who come to terms with each other rather than their respective spouses, who don't do much more than suffer from stupid past acts.
In comparison, of course, both Elizabeth and Darcy undergo subtle yet effective epiphanies. As various literary analysts have pointed out, Elizabeth and Darcy suffer from both pride and prejudice. The qualities are not limited to each character. (They both have a little of each quality.)Although Pride & Prejudice sets a standard hard to match, most romances don't require epiphanies from both partners--simply change and choice. Jane Eyre undergoes a more profound internal realignment while Rochester is rather forcibly brought face to face with his own mortality.
Arguably, a novel that doesn't involve the change of both protagonists isn't a romance. Rebecca is more suspense than romance--or it is romance in the older meaning of the word: a heroic adventure imbued with strong emotion. Little Women, on the other hand, is a family story, despite romantic subplots.
But a contemporary romance novel, even one with a historical setting, needs both protagonists to grow and alter in some way--hence, the plethora of current romances that use alternate chapters between protagonists.