One big change was that the assault weapons ban of 1994 only lasted 10 years. Congress let it expire. Afterwards, military style assault weapons became the weapon of choice for mass shootings. Quick trigger action and very high capacity magazines allow even poorly trained users to kill and injure a lot of people very quickly. Add body armor to keep the shooter alive, when security forces finally show up. Here is a
List of school shootings in the United States. That page is a rather long list that covers just those since 2000. There is a link to another long list for those that took place in the previous century. Most of these shootings only manage to kill a few people or even fail to accomplish that, but the frequency and lethality have gone up over time as more and more guns with enhanced firepower have become available to the public.