What is KARA's secret to their popularity?

For a group to last long, it is never good for one member to hog all the spotlight. If one member is noticeably more popular than the others, then it creates friction within the group and ends up having a negative effect to its longevity. Sadly, this is often the case in K-Pop groups - one or two members end up with all the popularity, whether they mean to or not.

However, that's not the case for KARA. Even though the girls had a scandalized break-up scare and Hara seems like she may be the only one popular, the girls share more or less the same size fanbase.

Seungyeon and Hara were active in variety shows, and command the bigger fanbases. However, all of the member-specific merchandise at their Tokyo Dome concert were sold out completely. When Seungyeon teared up expressing her gratitude of her fans filling out the entire Dome, the fans cheered loudly. The fans cheered equally as loudly for Hara and Nicole when they danced, when Gyuri sang, and when Jiyoung had her stage.

The fact that the members all had more or less equal popularity only means that they have a stronger possibility of lasting a long time.

Source: Herald Corp
Tip: Ronald Pacheco