Because it's a captive audience.
What Z1 said was spot on. The performance is not the issue - they need to go viral on naver/yt shorts afterwards to get the ULs, most of the massive promo has come from variety or other spots to songs. Sneakers, Forever1 etc are all hallmarks of this.
SNSD had 2 massively viral YT shorts and they shot up a rocket. They had fairly low UL base so had room to grown and expand.
Blackpink are not unknown like NJ - people know of them, people know who they are. Some performance is not going to give them any audience they don't already have.
Particularly VMAs. Anyone who thinks the VMA's is going to do anything is just huffing the copium.
Saying "All year promo help" is not correct. Still Life hit 900k ULs with no promo, Tomboy was like a goddamn rocket before (G)I-DLE even started promoting. Love Dive was on a similar trajectory. Look at their first few Days of ULs - insane stuff with zero music show promo (before they started).
Then, on the flipside - you have Loona, WJSN, Kep1er, they've all gone hard on music shows and basically just slipped from their starting position. Oh My Girl too. How do you explain all those groups promoting hard and getting nowhere?