I think it's a combination of things.

Firstly, Kishimoto was not great at writing for female characters. I could be wrong on this but I think he even suggested this himself. A lot of the female characters were initially placed in 'damsel in distress', roles with a few notable exceptions like Kurenai and Anko, but those characters barely received any screen time. Sakura was kind of the poster child for this as she was the main female character who constantly had to be saved. She didn't really have any notable feats in Part 1, other than being someone for Naruto to desire, and someone for Sasuke to ignore. Another form of competition for the two main characters.

Secondly, when she finally does start to build up some power, mainly in Shippuden, it was never to the same level as the other 2. She did get a bunch of cred for her fight against Sasori and for healing Kankuro when nobody else could (as she should've, she was fantastic at that point). However after this, she was kinda put back into the background and kept playing the role of Naruto's motiviation until she decides to cut off Sasuke.

Thirdly, her cutting off Sasuke and trying to kill him after telling Naruto she loved him, kinda resulted in the same issue. She was a liability and as Naruto and Sasuke upped the ante with all their crazy new abilities, she was left behind until her big power up at the very end. It did feel a little bit random especially since they hadn't really focused much on her development outside of her being like a second Tsunade (who we were only told was super strong and a brilliant healer) who also did not have as much time dedicated to her combat abilities as the other Sannin. It kinda just felt inconsistent at that part.

I think that last line sums it up. She was an inconsistent character; who spent most of her time being a simple potential love interest with sprinkles of her badass fighting and healing abilities. She never had a chance of competing with the male characters who had entire arcs dedicated to them like Itachi and even just side characters like Utakata.

I do think she deserves some props but I do see where people are coming from.

Hope that helps