Problem is, Koreans don't have a lot of choices when it comes to phones. You could either buy a Samsung phone, which are extremely cheap, have a big screen, nice colours and are overall good phones, or you could buy an LG which is seen to be inferior in every aspect. Then you have Apple and honestly, a lot of people do choose Apple but the price of a typical Samsung is so much cheaper.
So I guess a lot of Koreans buy Samsung because they don't really have another choice.
Re your comment about Koreans being image conscious and materialistic, I could not agree with you anymore. We are the most materialistic race on the planet probably. Not to mention proud.
So it's like: fuck you Samsung but thanks for making Korean a little bit more relevant in the tech industry.