I don't quite understand the point of this question. It should be obvious.
Because he's not an idiot. Telling the 3rd would have been the dumbest thing to do.
The Third hokage let Danzo be in charge of Root and part of the Anbu. He knew Danzo for years, over 50+ years and was his friend. Hiruzen knew what type of person Danzo was, even better than Itachi did. Even after Hiruzen learned that Danzo ordered the massacre, he did not punish him in the manga but let him happily maintain his position for many years to come. Hiruzen believed that both light and dark needed to exist in the village in order for their to be a balance and for things to work in the village. That is why he tolerated Danzo's actions.
If Itachi told Hiruzen, Hiruzen would just pull Danzo aside and scold him. Danzo would continue doing what he does, even if he was somehow demoted. He did things behind Hiruzen's back all the time. The bad thing is that Danzo would know that Itachi was the one who told Hiruzen, even if Hiruzen never told him...considering that Shisui was Itachi's best friend and the only other person that knew that Shisui was planning on using that KA for the coup. Danzo and his men would go after Itachi to get Shisui's KA.
I'm really glad he didn't tell Hiruzen tbh.