Sistar didn't sell well because of concept most probably. The concept they did got a lot of listeners but not hardcore stans. Same thing happened to groups with similar concepts.
Sistar songs didn't serve longevity as they were summer events but people didn't have use of them outside of it? Also 2nd gen songs in general lack longevity as downloads was bigger than streaming. Streaming is what gives longevity.
About covers i don't know
