First-person narration seems to be the fashion. See Fifty Shades, Twilight, and My Immortal to get the picture. Harry Potter has been extensively planned in detail and therefore doesn't count as a counter-argument. My Immortal isn't strictly a bestselling novel but it is still the most notorious fanfiction and has been translated into several languages.
Writing in first-person enables the reader to sympathise with the heroine, far more than 3rd person, because you can include the heroine's thoughts exactly. Could the intense difficulty of Bella Swan's clumsiness, her uneasiness with anyone apart from Sparkles and Fido be expressed convincingly in third person? I doubt it. Since bestselling novels often deals with Mary Sues a third-person Mary Sue would be detested by most readers jealous of the heroine's perfections. But if they saw her thoughts clearly they could surely be caught up in her world of fantasies. The heroine would take care to emphasise her flaws, whereas the author writing in third person would take care to emphasise the heroine's strengths, so 1st person is a good way to avoid MarySuetification.
On the other hand, writing that way makes the author become the heroine, so to speak, as it's so often a case of suppressed desires published in novel form. This is, however, not a great hindrance. An author writing out a perfect heroine in 3rd person would be dismissed as stupid, incompetent and a lustful middle-aged Mormon female. (Or anyone of any other faith). If you were to write a perfect heroine in first person, however, the discerning reader will explain it away as a case of a Narcissistic heroine with Schizoid Personality Disorder. This will make your novel look cool, profound and intellectual.
"But wait!" cries the fan of comedy. "What about different perspectives that make a novel funny?" This is quite easy. Simply make the heroine's perceptions of the people around her as ridiculous as possible. Writing everyone else as ridiculous is simply stupid in 3rd person narrative. With a silly heroine we can depict her thoughts as warped.
Love triangles and the heroine harbouring intense lusts for 2 different men at the same time are quite popular. In 3rd person she looks like an adulteress. In first person the intense longing, the unhappiness are clearly expressed in her voice. Never ever ever go for detachment, dear would-be novelists.