Lackadaisical writing by Kishimoto. We, the readers, should not have to make up theories for the things that have occurred in his manga, he should explain that perfectly, as it stands it's a plot-hole until proven otherwise. It couldn't have been Tsunade because that would be favouritism, and Hiruzen, the badass old man Hokage, never showed any of that nonsense to Naruto, so why should that crappy Hokage Tsunade show any to Sakura?
Especially considering the fact that one has to be intelligent and capable of leading a squad to become a chunin. Sakura has never showed any of those attributes in the series, on the contrary she's showed the opposite. For instance the rampaging Sasuke-vendetta-arc (five kage summit arc. Naruto and Sakura were pathetic so I call it the Sasuke-arc.) Sakura gathered up a team of some of Konoha's skilled members, searched for Naruto to lie to his face, took her team to hunt for one of the most strongest ninja in the world for her own selfish reasons, knocked-out her team, and abandoned them, confronted Sasuke, cracked under the pressure, and had to be saved twice.
Pathetic.
No chunin or heroine should be so incompetent like Sakura.
Kishimoto can't write females for shit.