Rouge had issues, they were the best in NA, yes, they won almost every game, but their play was bad, compared to the Koreans who played the game "correctly". The ult coordination was poor, they'd invest ults into losing fights, it was mostly evident with Dragonblades and Trances. They didn't have a good offtank, the only reason they did well was because they figured out how to run a correct dive, and they had so much mechanical skill, no one could compete, but they had many issues, it's just no one in NA decided to take the time and figure out how to beat them.
But when they went over to Korea, they got steamrolled because they weren't a team. Koreans value team play and having a team be worth more then just the sum of their parts, but Rouge were the opposite, so when they went over to Korea, teams disected them, they abused the fact that they didn't have a good offtank. So they killed the supports easily, while the tanks protected and pealed for their own supports and shut down anyone who tryed to do anything.
Rogue could only run 1 comp so they were very one dimensional. The koreans only needed to counter only one comp, and when they did, every team without a good offtank fell, Rouge, IMT, Renegades, Eunited, and probably more. Even though that is my opinion, Rogue weren't in the best of circumstances, they were shipped to a different country, stuffed in hotel rooms and forced to play well in a glass box. Pair that with the weak competition in NA and you have recipe for disaster.
So, to sum it up, Rogue lost because they didn't play like a team, their ultimate coordination and usage was wasteful and not planned throughly, they didn't have a good offtank and they could only play 1 comp, so koreans exploited these flaws and rolled them in APEX. After that they couldn't get back on the feet and 3 dps was much less viable after the koreans showed how to counter it.