Big drop in downloads starting in 2018, causing less turnaround on the realtime charts overall, but overall melon replacing the realtime chart with the 24chart is by far the biggest factor.
Koreans have been conditioned for over a decade to use the top 100 chart as a playlist, thats how most people discovered new music, by starting from #1 everyday and going down, most people never got further than top 10 during a day (Yoo Jaesuk have often mentioned how he is a typcial top 10 only listener).
Now, people either still use the top 100 as their playlist, and with how it work(ed) almost never hear anything new, just the same old. Others make their own playlist and mostly listen to the same favourite songs, as is more common in the west i think. In fact, the charts now looks more like the western charts in turnaround, while as before the korean chart stood out by having such a fast turnaround that songs staying for more than 3 weeks at #1 was extremely rare.
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I think looking at gaon digital chart on wikipedia and the songs with most weeks at the top on the chart says it all really:
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Pre 2020 only three songs ever made it over 5 weeks, and those three are some of the biggest hits to have been released in korea in the past two decades, true national hits as some like to call it.
2020 onwards, theres suddenly 10 songs that have done 5 weeks or more, with one song increasing a record that stood for 8 years at 5 weeks, by doing over double the time, and another five songs in total doing 6+ weeks. That in less than two years, something that took nine years for the first three songs.
