what make SNSD popular in japan

[chu-lips]

im curious what make them so popular in japan nowdays....
their sales even higher than many japan high-rated act such as koda kumi..mika nakashima...namie amuro,,
n even now they will beat ayu sales on yearly single sales....
dont make this thread as a trolling n bashing thread,,,,
i'm just want to know what's going on with them...n make them selling pretty high,,,
thx,,,