This is an interesting question, and definitely touches on the issue of modernity’s influence on the rise of nationalism. I recommend Benedict Anderson’s book “Imagined Communities” which discusses this topic at the global scale. One of his arguments is that modern nationalist movements largely replaced empires and monarchies, in which the sense of sovereignty was bestowed upon the monarch through religiously-ordained legitimacy. With new trends such as imperialism and colonialism impacting countries like Korea and China, sovereignty was no longer being thought of as belonging to the imperial house, but to the “nation,” a collective of people who shared a similar faith, mythology of origins, language, and culture. Korea, along with China, were such countries where this trend would’ve occurred. Also, with regards to the Korean imperial house, by the time of Japanese occupation they were seen by most nationalists as mere puppets of Japan, and hence had lost legitimacy as leaders of the Korean people.