Maybe Itachi found out that while others got mangekyou in the past, there is actually something stronger then that, and in order to achive it, you need to fight another mangekyou user and win...

So he destroyed the entire clan and left Sasuke so he can grow stronger and develop enough power to achive the mangekyou, and also told him the final secret that will unlock that ability when he is strong enough.

So basically:
1. Sasuke should get to oro, and get oro to get into him
2. All powerful Sasuke/oro will kill Naruto and gain mangekyou
3. Sasuke/oro should fight Itachi, and loose
4. Itachi will gain super-mangekyou-omg-wtf-2
5. Sakura will manage to avenge Naruto and Sasuke in a 60 episode long fight with Itachi
6. At the last scene of the whole show, Sakura turns to the 'camera' and says "SASUKE-KUN!!!"

the end.

sorry for the spoilers.