How? Ima talk fair and reasonable to you since you dont look like a fanboi bro.
If you were tobi:
Why would you kill some one that makes your group power stronger?
Kill someone who needs to die by sasuke and no one else? Remember tobi wants sasuke and he needs sasuke to be at his best with EMS, he cannot get ems without first getting Ms by itachi.
as you can see he needs sasuke to continue getting stronger. Itachi was in his plan of making sasuke become stronger aka getting ems
And right here is when tobi ask if he will transplant itachis eyes
And here is where he says he wants sasuke and that was basically his goal pertaining his plans. Do you see why he wouldnt kill itachi now?
Itachi was needed to make sasuke stronger Ms>EMS> we dont know yet. I hope you get what im saying Bro. Plus the fact itachi was sick and tobi knew this along with all itachis secrets being known aswell he could of killed him if he wasnt part of his plan. But itachi was part of his plan, so he left him alone.