Because it's ridiculous how you're making it sound like Sasuke is this easy gift lying around the corner for Orochimaru to pick up if by chance his plan of taking Itachi didn't work.
Itachi has gone rogue. He had no allies. Coincidentally joined Orochimaru's organisation and was assigned his partner. That's as much of an easy target as it can get.
Instead Orochimaru gave up on the Sharingan right next to him and decided to invade the most powerful village to manipulate Sasuke and have him seek Orochimaru himself.
His "easier option" was literally more risky and unlikely than just fucking going at it with Itachi. He was intimidated so much he completely gave up on even the thought of doing just that.
MarineHQ, what the fuck are you saying?
Orochimaru wanted Itachi's body for his sharingan.
Orochimaru says Itachi is stronger than him.
Orochimaru decides he'll go after someone else with the sharingan.
Why the fuck would he keep focusing on Itachi, who is in the organization run by Uchiha Obito (it comes off as Orochimaru & Kabuto having summed up most if not all of the Akatsuki's secrets) when there is literally another option?
If you want a beer, are you gonna fist fight your bartender for that beer or go to another bar where they'll hand it out to you without struggle?