Well for one thing as far as I'm aware nobody knew this was going to happen until ten minutes before the shooting, and I can't really blame people for thinking that he was just some edgelord looking for attention.It's more or less common sense making me wonder why stupid and insane people are allowed to talk about this sort of thing and not have people question them about it.
For another, the government can't literally be everywhere looking at what everyone's doing. Honestly I'm rather glad for that, even if shit like this ends up happening due to that.
Also it's worth reiterating what a user upthread already said: this man was not insane. He had logic, reasoning, and a clear mind with what he did. You can call the guy misguided, you can call him heartless, you can call him evil, and he's certainly a murderer, but he wasn't/isn't insane.