It's because of what you said: no motive. Crime like this is only sensational when the motive can be applied to some fictional stereotype of villain that could be stopped by new legislation or a war or whatever.
Also I think a big reason we don't discuss this specific event is the caliber of rifle used. Contrary to popular belief, non "assault" weapons can do a shocking amount of damage in an environment where the targets can't retaliate. See the Virginia Tech shooting.
Long story short: if it ain't political and can't be made political, people in the US won't care for long.
Turns out the dude was likely involved with CSAM, as his brother was arrested for it almost immediately after the shooting.
From what I've seen, his brother was one of the only people still in touch with him during his last days.