1 He liked the idea of having a uniform
When jobs visited Japan he liked the fact that workers wore a uniform. When many people at Apple rejected the idea of wearing a uniform he decided to have a uniform for himself.
2 He said it's convenient
According to Steve Jobs' biography by Walter Isaacson Jobs said that he likes to wear the same thing because its much more convenient.
3 To convey a signature Style
According to Walter Isaacson jobs said that the same clothes would convey a signature style and so he liked the idea.
4 Jobs loved simplicity
Jobs always loved simplicity and this was apparent in the design of Apple products and his almost empty uncluttered house. This might have led him to stick to one uniform. (See Why was Steve Jobs so persuasive?)
5 For personal branding
Some people suggested Steve did it for personal branding so that he gets easily remembered and recognized.
6 Jobs had almost a hundred of the black turtlenecks
Jobs asked the designer to make him something to wear and when he liked the black turtlenecks he asked him to make many ones and he ended up with almost a hundred. (See What are the greatest achievements of Steve Jobs?)