They maybe not everyone's cup of tea, but most people find it enjoyable
No one's asking everyone to like them
They're famous in Japan because nowadays girls/women tend to be more independent than only being beatiful and cute, so they could stand on their own and not having to rely on the man so much, somehow SNSD and other KPop girl group could give them an answer. Not too cute, but cool and stylish at the same time. That's why if you ask a Japanese guy about them mostly won't give a good remarks because they're idols for girls in Japan unlike in Korea.
I guess that's the reason why they're popular in Japan.
While in the other part of the world, I don't really know but there is a proverb in my country that goes like this
"Tak kenal maka tak sayang"
which means if we don't know or understand something clearly, then we won't know its true meaning/value so it'll be hard for us to appreciate it.
Most of my non-Japanese friends who loves SNSD are saying that they're adorable and they're cute in their own way. Maybe if we really want to know why they're famous and popular do try to know/understand them, that would be my answer
I did this too, I used to had negative thoughts over KPop groups, but once I know them all, those negative things vanishes