Because kishimoto had to nerf him somehow.
Itachi & Kimimaro both got sick because of Orochimaru planning to use their bodies , but without thinking his poisonous ill body would affect them in a negative way. Then he decided to make special preparations for Sasuke so Sasuke wouldn't get stricken with the same illness. Remember Orochimaru wanted to use both Kimimaro's and Itachi's bodies for his own.
NOPE. Kimimaro was on his death bed when he was introduced. He wasn't nerf'd in any way.
post: 619 booyaka booyaka Rey Mysterio!
Wait, Oro never experimented on Itachi. What are you talking about?
Kimimaro was strong af naruto had no chance especially when he summons 2000 clones and they all just stand there as other clones disappear, in his prime he would've been deadly but I strongly disagree w the top 5 statement. Part 1 Gaara almost defeated him while he was ill. I think ohnoki solos prime Kimimaro
he wanted to take his body so he probably used the hydra jutsu on Itachi before the Sasuke vs Itachi fight because Itachi said he remembered Orochimaru's Hydra jutsu "this feels like Orochimaru's hydra technique" (maybe not a direct 100% accurate quote) ,but Orochimaru probably used that on Itachi before and it infected Itachi with his illness.
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And? This doesnt mean much.
As for Oonoki soloing him, that may of may not be true. Both are top-tier. However, by feats Kimi takes this.