I think that the ice bucket challenge is popular because people think it's funny and want to spread support about ALS.

It wouldn't be popular is people thought it was boring. For example, if it was just text and it told you to donate it wouldn't have caught on as quickly as it did, or at all. If it was just people pouring ice on their heads people wouldn't be doing it.

The reason for the ice bucket challenge's popularity is that it combines fun with a purpose.