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.