Because the more mainstream it is, the less control the "they" (insert correct term here--damned if I know who "they" are but I am certain there is a "they" that desire servitude from as many people as possible because that's the game) have over us.
Knowledge used to be something kept fairly secret because in the past it was hard to research how to do and make things. I used to have a lot of trouble finding information about my cars in order to work on them. The same experience repeats--you had to be in a club with like-minded people to bootstrap knowledge and people held on to manuals and homebrew documents created on a typewriter with annotations in the margins. Now all that knowledge has been dumped onto the Internet and is easy to find and videos make it fast to pick up.
The problem with being in a club is that most of the personalities don't mix but the club controls the gateway to the knowledge. For a long, long time, gun clubs were loaded with Elmer Fudd types who looked down their noses and modern semi-automatic rifles and handguns. They only accepted typical hunting rifles and shotguns, the odd Browning semi-auto, maybe an M1 Garand or M1 Carbine for WWI and WII nostalgia, and of course, the cult of the 1911. But they didn't understand the 2nd Amendment, not like many of us do now. Because they didn't need to--there weren't vast armies of thug leftists trying to bludgeon the nation into some form of corporate collectivism.
What has changed is manufacturing technology, not the guns themselves. Things have changed quite remarkably and those old ideas are disappearing, which is mostly good. What isn't good is that the knowledge, information, and yes we have the best selection of small arms in 150 years, is available to all at affordable prices. A few outliers are having problems dealing with the responsibility of it all. But we as a society can't cater to the least common denominator. We have laws to deal with criminality but lack the will to do so--hence why the Broward deputies didn't arrest that wacko during one of the 40 some times they were in contact with him. Either they directly desired his violent outburst for political reasons, or they were just too politically correct to act. Regardless of the details of each incident, we have a remedy in law and that is to punish the guilty and stoically move on. If the sheriff were to lose his job for being incompetent, all the better.
Firearms are just one teeny part of what it takes to stop the oligarchy. Humans tipped the balance once before: When guns were invented and first arrived in Europe, commoners made good use of them shooting armored noblemen right off the backs of their horses because screw those guys. The scientific revolution being the direct result of the end of medieval period. We all owe a lot to firearms, they are the necessary evil for humanity.