A lot of reasons which compound and increase the likelihood. (In no particular order:) It's not a coincidence that the same liberals who think that babies (especially black babies) SHOULD be aborted, much less may be aborted, have no problem killing other people. It's not a coincidence when 95%+ of all mass shooters are democrats / liberals / socialists, and the people who killed 150,000,000 people in the 20th century are also democrats / liberals / socialists.
Then you can take into account the fame and notoriety that these people get. Even those media outlets that refuse to name the person who does this thing have spent DAYS talking about it. The social isolation, and the negative consequences of the socially isolated talking to each other on the internet and NOT getting any kind of feedback on their ramblings and psychotic thoughts is also amplifying the problem.
Next you have the failings of the FBI and other police organizations to actively investigate ANY threat, much less do something about real threats. Light punishments or the refusal to punish minor infractions in the name of compassion or anti-racism leads to larger infractions later. Of all of the shooters, excluding this latest one, the FBI or the police literally did nothing when the shooters made terroristic threats or other things. Then later they EASILY passed the background check because they had committed no crimes.
It even goes further than shooters. Trayvon Martin was found in possession of $5,000 in stolen jewelry while at school. Had Trayvon gone to jail for felony grand theft, instead of the school COP being told that he "just found the jewelry", Trayvon might still be alive today.
I've even heard talk and musings about potential future serial killers seeing the advance in criminal sciences leading to more serial killers getting caught easier, so they decide to unload their impulses in one big bang rather than slowly over time. Hell, the Golden Gate strangler got caught AFTER he stopped not because of something he did, but because his sister took a genetic heritage test.