Bump stocks were attached to 12 of the rifles found in the Las Vegas gunman's hotel room. The modification enables semi-automatic weapons to fire faster. The National Rifle Association said recently that it would support a restriction on bump stocks, but that it opposed an outright ban.
After the shooting, YouTube said it took a closer look at the videos in question (that teach how to adapt semi-automatics) and expanded its policy to cover such content. But it hasn't been as quick to take the offending clips down. A search for "bump stock" on the site returns hundreds of thousands of results with titles such as "home made bump fire stock ... for less than $20" and "Upgraded Rifle Efficiency How-to Part 1."
It's important to note that most of the recent videos on the subject are from news channels discussing the role of bump stocks in recent shootings and whether they should be banned. Still, there are plenty of explainer and tutorial listings that haven't been taken down, and that sheer number could be what's causing the delay. YouTube reacts when clips are flagged as violating its community guidelines, and its global review team evaluates and removes offending content. To make a dent in the pile of videos it has to remove, YouTube will probably need to rely on its users to proactively flag such media, and that will take time.