It's because the US right will celebrate literally any action that they perceive as working against what they think everyone left of them supports or enjoys. Kyle was a clean cut looking young white man who heroically skirted the edge of laws regarding firearm purchases and visited a town that was not his own where he made sure to keep looking until he find a situation that required him to use his gun. The context was protests fueled by the death of George Floyd and shooting of Jacob Blake at the hands of police.

There were probably folks who literally touched themselves after hearing a red blooded, AR wielding young white man was able to be acquitted of murder after shooting protesters at a BLM protest. On top of that, one of the men had some form of pedophilia in his past, boosting their drumbeat of messaging claiming that folks who support LGBTQ+, and by extension all democrats and leftists, are groomers out to molest kids.

It was a perfect storm of trump supporter daydreams all centered around Kyle Rittenhouse. Folks who buy into all or most of that view are big fans.