Why was Linn made a backup singer in the group Ace of Base?

Ace of Base was a popular band. Linn (the blonde one) was the lead vocalist in the first album. She had the better voice. All of a sudden, Jenny (the dark haired one) was made the lead vocalist. How come?|||"Jenny was quite unprepared for the spotlight; fans have described her early television appearances as resembling a "deer in the headlights". Later, with sister Malin's sudden withdrawal from the spotlight, Jenny was forced into a more prominent role." more info at this site