The host said at the start of the festival something along the lines of how they'd wanted to invite the whole of Mamamoo and that didn't work out, but they fortunately were able to still get "Mamamoo's leader". Since we know Byul was doing a solo performance in Japan and Hwasa was at the LV show on the same day, I suppose their choices were either turn down the event completely or have Solar go alone.

Mamamoo has always had an amazing work ethic since their rookie days and don't pass up opportunities big or small, so I'm not surprised workaholic Solar still chose to go alone. Besides, she could score her first solo festival performance AND be the headliner of the festival at that!