They are very expressive
Memes are expressive images that people can use to quickly express their emotions or their concerns about something. The fact that memes can be used to express almost any emotion or to comment on any event made them very popular.
A very fast method of self-expression
People use various ways to express themselves including writing posts and status updates. Memes are among the fastest ways people use to express themselves, which is one of the reasons they became widely popular.
They are usually Funny
Memes are usually funny and can sometimes send an important message in a funny way. Because they are a lighter form of expressing one’s opinion compared to serious talk, they became widely popular. (See What makes a joke funny?)
They spread fast from one person to another
Memes spread fast through social networks. A person who saw a Meme for the first time can recognize its purpose in seconds and ends up sharing it as well.
They are based on images
People process images faster than they process text. Because memes are based on images, they are faster to process and are easily noticed, so they are more likely to spread fast.
Many of them are based on popular events
Some popular memes have their images taken directly from a popular event such as a popular movie or a TV interview. The fact that people can identify the image fast helped memes become more popular.
The pace of life is changing
The world is becoming more fast paced with every passing year. Memes are a very useful tool because they match the speed of the new fast paced generations. (See Why some people want everything fast?)
They are relatable
Memes can describe a life situation that a person has been through. The fact that so many people relate to memes helped them become popular.
They are easy to create
Memes can be created fast due to the large number of free tools and apps that allow people to do them. A person can quickly pick an image and write something on it, thus creating his own meme fast.
Some of them are emotionally provoking
Emotionally provoking content usually gets shared more often. Because some memes are very emotionally provoking, they get popular fast.