The "problem" with Sakura is that despite being a main character (if this was a live action show her name would be in the opening titles) she is not as developed, interesting or impactful as her teammates Kakashi, Naruto, and Sasuke. Hell, we've never seen Kakashi's full face and we know more about his life, his goals, personal failings, family history etc.
Looking at the story of Naruto and Shipuuden, the main arcs are about Naruto and Sasuke. Naruto a talentless ninja, who through hardwork, perseverance and assistance from both his teachers (Jiraiya, Kakashi, and Iruka) and Kurama (by virtue of being a jinchuriki) has forged himself in to one of the most powerful ninja on the planet. Even before he was god-level, Naruto had a history of fight physically superior opponents (Kiba, Neji, Kabuto, Sasuke, Orochimaru, Kakuzu, 6 Paths of Pein, Third Raikage, Muu). Where he defied the odds and came out victorious or inflicted serious harm to them. Naruto is the main character and hero of the series and has been intent on ending this war and bringing peace.
Sasuke a ninja born with incredible talent, who pursued his cause to avenge his clan and kill his brother by following a path of darkness. Sasuke was mentored by Orochimaru and Kakashi and has continuously improved fights. Defeating kage level opponents like Orochimaru, Deidara, Itachi, Killer Bee/Habachi and Danzo. Sasuke has given up his quest for revenge and now seeks to end the war and protect the Leaf village in his brother's memory.
Kakashi is the sensei of Team 7. While one would imagine he would have the same pitfalls of underdevelopment as Sakura, in the shadow of Naruto and Sasuke. Kakashi avoids this by having a back story that connects to the present war arc and by having good shows of combat ability against dangerous opponents throughout the manga (Zabuza, Sound Ninja, Deidara, Hidan, Kakuzu, Deva Path, Sasuke, 7 Mist Swordsmen, the tailed beasts, and Obito). Kakashi's connection to Obito, his former teammates; gives him clause to be involved in the war arc outside of obligation to his nation. Obito was manipulated in to crafting this war and the Akatsuki because of what Kakashi did to Rin back when they were kids. Kakashi's story is woven in alongside Naruto and Sasuke's.
Finally to Sakura, she is the least developed of the main cast of Team 7. She has no arc (like Naruto and Sasuke), or undercurrent of a story (like Kakashi) to speak of. Her presence feels obligatory. She has skills, but they are not put in to practice to help move the story forward. The exception being her fight with Chiyo against Sasori. The arc/story she does have seems to revolve around the affections of a man (Sasuke) who is not interested in her. If there was reciprocity in this relationship, maybe Sakura's character wouldn't feel so stagnant, but there is none. The Byakgou Seal power up was revealed out of left field but you can accept it because of who her sensei is, and it was way past due for Sakura to gain a power up. However that growth is negated by Sakura having to be rescued by both Naruto and Sasuke a few pages later in the chapter and Naruto and Sasuke gaining an even more hax power up to their already OP arsenals. Sakura can't win for losing. If she was a minor/supporting character like Ino, Hinata, Shizune, this wouldn't be such a big deal, but she isn't. Sakura is supposed to be just as important as her teammates. It's Naruto's story so he's taken care of. Sasuke is Naruto's rival and a sizable chunk of the story revolves around him. Kakashi is connected to Naruto, Sasuke, Obito, Minato, Guy and Sakura. This gives Kakashi enough clause to merit his prevalence next to the 2 mains. Sakura feels like a tacked on companion by comparison.
