1 The word ninja became a compliment
The word Ninja became a compliment that describes a person's mastery of a certain skill. So many people use the word Ninja in their internet nicknames, blog posts or websites.
2 Unconscious desire for power
Most people wish to be strong, powerful and omnipotent. When a person sees a ninja and identifies with him this gives him a sense of power and strength. Our unmet needs determine the things we love to a great extent.
3 They are rebels and outcasts
Ninjas don't follow rules; they are perceived as rebels who live by their own rules and abide by no ones rules. People who are rebels by nature and hate to follow rules usually find ninjas very appealing.
4 They are in control
Ninjas are always in control. They jump over obstacles, climb over anything, are very skilled at martial arts, know how to disappear and can hardly be stopped. Those traits all together make ninjas so appealing to a wide range of people.
5 They mastered many skills
Ninjas have so many skills that they do perfectly. People look up to them because unconsciously they think that they are great examples of mastering the needed skills in life.
6 Ninjas are mysterious
Unlike normal guys who practice martial arts, ninjas are mysterious and unpredictable. Mysterious heroes can be very appealing since a person's imagination can make many conclusions about them without being blocked by facts. (See Why Do We Like Superheroes?)
7 A ninja is a very general term
A ninja can be a hero, villain, a brave warrior or even a super hero. Because the word ninja is a broad term that doesn't describe a single persona, ninjas became widely popular since they appealed to so many people.
8 Some ninjas turned into super heroes
The ninja turtles, Batman and many other super heroes are ninjas. This helped the ninja concept become very appealing to a much broader audience.
9 There are very interesting variations of ninjas
There are so many interesting variations of ninjas. Scorpion and Sub Zero, the famous Mortal Kombat game characters, are among the most interesting ninjas that so many people like.
10 They are superior to humans
All humans want to feel superior and Ninjas are usually presented as superior humans. It's not unusual for a ninja to kill hundreds of people in one ninja movie. (See Why do people strive for superiority?)