Part of the reason why, is that it's contagious. The more mass shootings that happen and the more they're publicized, makes it more likely to put the idea in the head of the next maniac. If there was no incidence or news of mass shootings in the USA for the next 10 years, the chances of one happening in year 11 would be a lot less than it happening next week, while news is fresh. Ex. Columbine helped lead to many more school shootings.
Also firearm technology has improved over the years, which is a factor in making the incidents more deadly