"So, we,re getting a two-fer - a mystery AND a romance! Like Dickens' Edwin Drood!"
And the tradition continues. Many modern mystery story writers start out as romance writers, then move over into mysteries, but always provide a romance (Janet Evanovitch is a popular example).
Actually, (and this may be more than you want to know) there are subgenres in mysteries, and the ones called "cozies" (think Agatha Christie) usually have a romance in them. Collins is perhaps the first "cozy". They aren't as dark as other mysteries, and are "cozy" in that you get to know and like the characters, feel you could settle down with a cozy cup of tea with them after the murderer has been hauled away. They are often funny and light, as opposed to the "psychologicals" (think "Girl with the Dragon Tatoo") which are full of pain and angst.
There are other genres, too: procedurals, tough guys (or gals) etc. and other ways of classifying them, but enough of that.