i think it's becuz the digital downloads become more and more popular just like in us
Exactly. For example, IU's signature song "Good Day" was #1 for 4 weeks and is still charting in the top 200 til this day, but it only crossed 3 million some weeks ago, while You&I became the best-selling overall in 2 months. Other good example would be Secret. Shy Boy is easily their biggest hit, but it only sold 2.2 million, while Love Is MOVE sold 2.6 million.