Is because they lack charisma on stage. notice how the most popular groups internationally are ones that are known for their performance skills.
I remember during the 2nd generation, even though the groups had mostly basic choreographies they still had charisma and looked like they enjoyed themselves on stage which automatically makes people enjoy the performances more, this also coincided with SM groups being at the top both domestically and internationally. Up until bigbang blew up with Fantastic Baby, tvxq and suju had the biggest touring power and snsd goes without saying.
But then things went down hill sometime during the 3rd generation. exo had all the necessary technical skills but their performances were hit or miss, rv were so dull on stage, nct has some charismatic members but only dream look like they truly enjoy performing while the other units make it look like a chore, and now aespa are practically the least charismatic group in SM's history.
what went wrong exactly? Is it an increased focus on perfection that killed their idols' individuality and passion?