what makes spongebob squarepants so popular?

Well, not necessarily. Many people like SpongeBob because he, along with his friend Patrick, are funny characters. The show is for kids, so obviously it's going to be a comedy -- for kids. He's probably popular because it's the only funny show LEFT on tv these days...

because they're funny and they don't even know that they are

What's not to love? That yellow piece of square makes me laugh all day. It's his attitude that makes people like him so much and he makes people laugh a lot. The kids like him also and as you can see from my avatar. I'm a big fan even though I'm 19 and I'm not ashamed to say it. There are times where I snuggle in my Pajamas and just watch Spongebob all day.

Positive attitude plus he's funny. That would be my guess anyway...

He's plain way of thinking and the laughter also, quite a hilarious guy.