it is the result of many piled up causes. liek for exemple, some members' relying so much on variety which shed a bad light on the group's musical identity. especially that at a certain point in their career, the most popular members were the variety line so it made it look like all the group was about is variety.
then there was Sorry sorry which was YYJ's biggest hit so he kept trying to recreat it and SJ's image suffered dearly because of that. now they are known as the group whose title tracks are remakes of each other thank you very much YYJ
and then there is all the bad marketing from SM. they never debuted SJ as a serious group so everything SJ achieved was treated as a bonus and they never treated them like their other groups. they never highlighted the group's musical talents or skills like other companies do and did nothing to redeem the group's image, in the contrary, the kept doing one reckless move after the other killing the group's local popularitya nd potential with every step and now it reached the point where the public are done with them and no matter what they release it will flop regardless of the quality but because people simply couldn't care less (and in some cases aren't even aware thanx to SM's marketing or the lack thereof)