Well because he's the best written villain in the series. It's not about raw-power or shinny jutsus. It's about mind-set, and his unique charisma.(I mean everyone got their own reason, and they were good in their deep heart. Obito, Madara, Sasuke, They are all same as Naruto but their way to achieve their goal was way different.) He doesn't care about anything. He's just care about himself and his goal. How to learn everything. It seems it's really good goal. Want to know every just want to understand everything but what cost? That's the part. He doesn't give a fck about getting his hand dirty. He's not strong like main char or last villains but he has always a back-up plan and he knows his place. If he had to he makes allies and try to get advantage of them. Sharingan is his true goal or short-cut for learn everything but Kabuto already explained, Orochimaru already learned or guessed, If he can obtain a true sharingan or uchiha body. He can able to achieve Rinnegan. It's why he's a uchiha fanboy and he doesn't care about it too. He already give-up his mere human self. Turned into a snake. I was just hoped for he would be a final villain in series and it didn't happen. I still got hopes for him.
