Seriously
Apart from what I already mentioned there are also other factors. People are dismissive of opponents or inevitable they don't like. Denial and bargaining are the run up to acceptance. 'KR will never catch up' became 'we can beat CN'. And partly, the latter is true. CN just didn't have the strategic depth, diversity(strategic diversity is not bloody champs played before someone downvotes me again. It is what comp designs you run-poke/siege/wombo/splitpush[1]/1-3-1/protect-the-hypercarry too but mostly how you utilize them to achieve your goals by choosing what to contest and what you prioritize this includes fighting over objectives, prioritizing early B/D or towers, side wave prioritization, jungle pathing and focus and stuff like that. Picking Poppy doesn't suddenly create strategic diversity) or vision focus which KR had. Mandu/Mata level warding just wasn't seen there and the games didn't feel as clean. There wasn't the innovation which KR had with things like the SKT ward or laneswap plays.
Of course, none of that detracts from CN being a strong region, just not at the same godly level at which KR had been. Pros are often publicly dismissive of their opponents either for hype or as a display of confidence so that itself added to it, with many LCS pros not even following KR/CN earlier.
Personally
If the Korean exodus didn't happen I would personally have cared too much for China. I care about LCS because of some emotional investment(can't find a better phrase) in the scene and certain teams. The shared language and relation to culture and players exacerbates this and that is a great reason why NA is financially stronger than EU. Fans care, investment happens and talent is procured if needed. Further, I love watching certain teams or players play, mostly C9/Shook(when they were ALL)/Lustboy and the vast availability of streams makes the scene relatable. LCS is a more spirited fest with the focus on story and cheering than actual analysis. I watch OGN because I love the game and I get to see it played at the highest level. Watching early game strats, jungle movements, interesting comps and a million other things executed cleanly is breath taking. I loved OGN but after the format change and exodus it doesn't feel the same. There was 0 reason for me to care about China when I can watch the best region or the most fun-to-watch regions, with it not meeting either criterion. If I was to instead spend time to watch a lower quality broadcast not cast properly in English I'd need a bloody good reason. KR losing people like Kakao/Dandy/Mata/Spirit/Deft suddenlt made me care about CN.