It is very funny
The show is funny. The fact that it makes both children and parents laugh made it more popular among people.
Even though the Cartoon is popular among kids still it contains lots of political jokes. This made some adults like it especially in the united states.
The show contains a lot of Satire humor that discusses many of the issues inside the united states. This made many adults like the show.
He is innocent and happy
Spongebob is an innocent and happy character. He sees the positive in everything and this makes him more appealing to some people.
His face is expressive
Because the face of Spongebob is expressive his memes became popular on social networks. This gave the character more exposure. (See Why are internet memes so popular?)
The characters of Spongebob are novel and unique. Unlike other cartoons which might have ordinary looking characters Spongebob characters are considered novel and weird.
Many of the scenes of Spongebob were memorable especially for kids. Many of those scenes were also easily converted into memes which made Spongebob more popular.
It has been around since a long time
Spongebob came to television in 1996. The fact that it has been around for a long time made it more popular.
Many of the people who grew around Spongebob remember their childhood when they see it. This is why it became more popular. (See Why Mr Bean is popular?)