if orichimaru was looking for immortality than why wasn't he interested in hidan or kazuku? there are also a whole bunch of people that he could try to possess that would be better candidates but why was he so obsessive about itachi?
The problem with him keeping the eyes is that the person who takes it has to be bloodpaired to the sharingan for it to work. That's why everybody was surprised, and Itachi mentioned that it was lucky that Kakashi was compatible.
That being said, I've always wondered it too, about him not aiming towards like Hidan, since he actually has like true real immortality...
I have laid my dreams under your feet;
Tread softly because you tread on my dreams.
^ Hidan's Immortality centers around his faith and his ability to kill(or at least the data book says so.). I would think that it's a bit cumbersome to have to pray and kill every once in a while in order to stay immortal. Kakuzu didn't really have immortality in the same way hidan did, he was just too hard to kill. And he still has to reload on hearts once they give out.
That and the sharingan is hax.
Another problem that would occur after sasuke's body tired out and oro had to transfer his eyes to a different host: it might end up like kakashi. It'll be stuck on, and drain his chakra like crazy.
Because Orochimaru is immortal, the White Snake grants him the ability to transfer his soul to other bodies.
Hidan requires to insist with his ritual to perpetuate his immortality or else he´ll become a normal again and Kakuzu has his hearts method.
He´s not interested in either of them become their own method of immortality are as good and as flawed as his own method, although Orochimaru´s method is still superior, since he probably escaped from Itachi´s Susaano, if the escaping snakes are any judge. I doubt Hidan and Kakuzu would be able to do the same.
After failing to destroy Konoha, Orochimaru states that if he had been able to get Itachi to join him he would have had no problem destroying Konoha.
As others have mentioned, Orochimaru already has a method of immortality that is equally as effective as Hidane and Kakuzu's. While there methods have certain advantages, they also have disadvantages. With Orochimaru's technique he is able to sustain his youth by taking a younger body and also obtain new abilities through his host. Also, his white snake body allows him to survive a vast amount of damage (perhaps even more so than Kakuzu). Kakuzu's abilities and Orochimaru's seem to be very similar, both are capable of surviving massive amounts of physical damage and both take abilities from others. Orochimaru probably wouldn't gain that much by taking Kakuzu's abilities (assuming he even could). In the case of Sasuke and Itachi, if Orochimaru had been able to take either of them he would have gained an immense advantage over his enemies and a way to quickly obtain jutsu, this could eventually allow him to discover an even better method of sustaining himself (after all Madara seems to have achieved a method of immortality as well).
Why should Oro look at someone else's method to be immortal or live a very long life, if his method is superior?
Which I think it is. Especially if you consider his goal...to learn all jutsu. Hidan and Kakuzu's method wouldn't have given him the sharingan, which you pretty much need if you want to learn all jutsu (execept bloodline and jutsu without seals...like rasengan). Without the sharingan it would take forever to learn everything the normal way.
With Oro's method he can have it. And I think he normally wouldn't have to worry about the eyes for the next transfer.
I think the time his body rejected him was still the after effects of what the third did to his arms.
Normally the bodies probably don't reject him, or at least not that fast.
Because if it's normal that he has to switch every few years, being so hell bent on the sharingan makes little sense. Unless his plan was to take the body and make lots of babies to have more sharingan users in the future.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"
I really believe that there were better ways for orochimaru to gain immortality, he just got obsesed with getting the sharingan...
Orochimaru still lives inside of Kabuto the kabutomushi