Couple different factors there, but it mostly just comes down to some easily explainable things. A shooter without a motive isn't a story that sells well, and it isn't a story that people generally want to read. Your highest profile american crimes tend to be perpetrated by extreme weirdos. I think it's probably just that this guy was kind of a sad old dude, and probably a pedo to boot, so it doesn't really make for a nice, harrowing story. It's just depressing, mostly.
Most readers, I think, want a kind of, narrative, or meta-narrative, around their media consumption. You can see people in this thread, trying to stamp one onto this shooting with the whole bump-stock thing, which I think is mostly just a minor aside, but for the fact that it kind of ties into a larger narrative about gun control, a larger meta-narrative, that serves political ends. Even in that, though, it's not a very good grafting subject for those stories. The fact that it was passed by a republican president means that it can't really serve mainstream political party end-goals, and bump stocks aren't really a significant concern, despite how people might want to make them out to be. Basically their only tactical use case is something like this, otherwise, they're mostly a toy. They don't really have the same use-case for gang violence, like you might see with glock switches. So they don't really present a highly defensible instance of gun control going wrong, and they don't present a high-priority target in terms of gun control legislation.
It is almost impossible for most places to do reporting in a way where you are ever given the full scope, the full picture. It's hard to report sobering data which might give you the larger picture, because it's uncertain, up for contestation, boring, and unrelatable. It's hard to report on everything in an indiscriminate way, if you're just reporting everything without any bigger picture questions, then you're liable to simply serving stories with no external context that would ground the reader, and you lead the reader to only ground themselves. If you do this enough, in combination with the A-B testing that might tell you what to actually report on, you'll just end up becoming 24 hour nightly news, where you just report on murder and rapes and serve political agendas without any real knowledge of what you're doing. Things have to inherently be passed through the filter of a meta-narrative in order for them to make any sense, to have any meaning at all. If you can't really do that, if all you're left with is meaningless violence, you will probably just see people ignore it.