It is very simple to play
People love simple things. The simpler a product is, the more likely it is to become popular. It is extremely hard to reach the max level in Maple Story but extremely easy to learn. It has almost no learning curve and after a few seconds a person can become aware of all the game controls.
Supply and Demand
When the demand for something rises, it seems more valuable. Since many people played that game, it is in high demand and hence, appears valuable.
Interesting looking characters
The characters are designed with big head and small body. Hence, they look cute. Humans are designed to find things with large head and small body cute so that they will instinctively protect babies.
As the characters level up, they can add in new skills or even go through job advancement and hence, a sense of novelty.
Self esteem boost
Whenever the characters level up, the players will feel a sense of accomplishment which boost their self esteem.
Life is full of stressful events and one of the best things a person can do to defeat stress is to escape to another world for few hours. By escaping into a virtual world a person can escape from his daily stressful life.
The game is free to play. This gave a certain age group ,specifically teens, a free entertainment method that requires no money. (See Why is Cut the Rope so Popular?)
Low resource requirement
The simple graphics of the game makes it run on any normal device and so even those with low performance devices can enjoy the game.
The social element
The social element makes addictive games more additive. The fact that Maplestory is an online game allows players to interact with others and so get addicted to the game.
Many ways of interaction with others
The game offers many ways of interaction with other players including chatting, trading and playing mini games. This made the game more interesting for more players.
Resemblance of Anime
The characters of the game resemble Anime characters to a certain extent. This could have made many Anime fans like the game. (See Why Do People Love Anime?)
Why was Maplestory so popular?
Why was Maplestory so successful? How did Maplestory get so popular?