old songs that chart more than 52 weeks. OP: "I just observed while reading that digital thread. Chart reforms are not excuses since IU has old songs that chart more than 52 weeks."
I feel like I'll regret reposting this thread but I was about to write a way too long answer for it to belong in anon. Here I'll at least get three akorns. 
Disclaimer: some of the numbers might be outdated by a year or so (hopefully they aren't) since my sources are wikipedia and old tweets.
IU has better longevity because she has better digitals period. She's the digitally strongest artist in Korea and there's no two ways about it.
And the question shouldn't be why Bigbang has worse longevity than BTS but why BTS' longevity is so off the charts (or "on the charts" lol) good. BTS has unproportionately good longevity. In comparison their short-term peak achievements are really bad. When I say "bad" I don't mean actually bad, their numbers are great, but BTS has the best longevity out of any Korean artist, including IU and non-idols, while their peak charting is similar to other top groups. For example BTS has 4 songs that reached #1 on Melon. IU, the artist with the most #1s, has 26. And Twice has 9 songs is that reached #1 which is the most out of all 3rd gen groups. Winner has 4, same as BTS. And iKon, Blackpink, Red Velvet and Mamamoo all have 3 songs. Overall no big differences between them and BTS. But BTS has the longest charting song in Melon history with Spring Day which has now been on the top 100 for over 5 years. BWL lasted over three years and they have 6 other songs (I think. not really sure) that charted for over a year. In comparison IU's song with the best longevity, Blueming, is still shy of 3 years and even though she has almost 7x as many songs as BTS that reached #1 she only has a total of nine songs that charted for over a year, very similar to BTS. And no other idols groups have more than three songs that charted for over a year (and that's only Blackpink). While less extreme you can notice the same pattern if you look at BTS' unique listeners. BTS has the two idol songs with the highest UL of all time and two (maybe three) more on the top 10 (this obv goes hand in hand with their longevity). But their ULs in the first 24 hours aren't nearly as strong. Blackpink, Red Velvet and Mamamoo all have songs with higher ULs. So does Jennie and Mino's solo debuts. (and a bunch of other artists but I'm trying to stick to 3rd gen idols for a more accurate comparison) I'm not sure why BTS' charting look so different from other artists', someone who knows more about how the charts work might be able to explain, or it's just one of those unexplainable BTS-phenomenons, but using BTS' longevity, while amazing, as some sort of default doesn't work. BTS is the (very impressive) abnormally here, not the other group.
I lowkey had really fun writing this. Got to look at so many charts 
So to summaries, Bigbang has worse longevity than IU because her digitals are better than theirs across the board. BTS has better longevity because they are doing BTS things and their charting doesn't work like others lol.
I kinda get why this anon thread was made. Bigbang is the group with the most #1 songs on Melon and also has the highest number of unique listeners (depending a bit on how you measure it) but they don't really appear on lists over songs with the best longevity. But yeah, hate to disappoint anon OP, it's because of chart reforms. 2nd gen groups in general had better digitals than 3rd or 4th gen groups yet all longevity rankings are filled with 3rd gen songs (and 4th gen will most likely start appearing soon). If you instead look at lists over groups with the most #1 songs they'll be dominated by 2nd gen groups. The reforms have made the charts more stagnant. It's harder to debut on them but once you're on it you'll stay there for awhile.
