I suspect but cannot prove that the mainstreaming of sound suppressors is largely due to the fact that information detailing the legal process for acquiring NFA items became widespread via the internet starting in the early 2000s.
Once upon a time, few people were willing to jump through the hoops to acquire an NFA item because just learning the process was considered to be a daunting undertaking.
That has been a big part of my belief.
For most of the time since the creation of the NFA people had to first learn it was even an option to legally own, then they had to be interested in learning how to do it.
This used to require either already knowing someone that already knew the process, or ordering books or looking for details in some library.
Even if someone found the details they had to be sure the information was reliable, what some random author says on a law even today can be wrong.
So once learning the process they would want to double check it.
Today people all learn about it and exactly how to do it and that many others have done it, without even trying.
But there is another big one:
Another big change I imagine contributes is a greater acceptance of big brother.
I know not that long ago a large number of firearm collectors were suspicious of government knowing about their firearms, and the NFA is registration.
Governments around the world have long used registration to know who has what, who is the type likely to have weapon "stockpiles" they don't know about based on obvious interest in what they do know about, etc
The GCA of 1968 started FFLS and transfers through them, but person to person sales with no records were still quite common (and still happen in fewer states today).
Even California had private sales until the 90s.
Even after the GCA the records were just paper, large quantities of paper hard to even sort through, the potential of computerized databases didn't even exist until computers became common. There was no checking in with government, or much monitoring.
Now there is NICs even on basic purchases, requiring checking in with big brother to buy any gun.
Even Concealed Carry licenses contribute to the mindset, paying a fee, getting government permission, and then being allowed to do something with a firearm.
Our whole culture makes people more accepting of less privacy and the idea of checking in with big brother and being registered. Things that I am sure didn't appeal to as many people in the 60s-90s.
There is also extensive security screening in airports, sporting events, courthouses, etc people are used to emptying pockets, taking things off, and having to please government or security to be allowed to do something.
Such screening primarily started in the 90s (metal detectors in the 70s at airports) and ramped up in intensity post 9/11.
People are also used to being on constant surveillance cameras everywhere, cameras were once rare and expensive, primarily limited to high security locations.
Look at the widespread use of various websites where large numbers of people put huge amounts of personal details, profiles, pictures, etc
While all these things may seem unrelated on the surface, it creates a different mentality in society in general.
People have become used to others having access to lots of information on them, asking permission to do things, and become more comfortable with being tracked by government than I think in prior decades.
If you had told many in the 50s-70s, or even into the 80-90s that they would have to ask permission, be registered, pay a larger fee, and let us not forget check in with government to cross state lines, I think many would have balked at the notion.
In a post 9/11 Homeland Security state with widespread technology people are desensitized to a lot of things that were big deals to some before.
Take it a step further, an individual is also at the mercy of the ATF to properly maintain their records of legal procurement while the individual is known to have items subjecting someone to large sentences for mere possession without the proper paperwork.
A less than honest government could 'misplace' or lose such records, especially when they were all just paper records locally filed.
Placing all burden to prove legal ownership of a restricted device on someone with 'forged' or 'unvalidated' documents.
We might take for granted that won't likely be done in the USA, but you need only visit Mexico to know other places are not like that. Many other governments past and present around the world could readily misuse any such opportunities.
So both the widespread sharing of information through the internet, and a significant desensitization to many things that would have bothered people in prior generations I think play equal roles.
Today asking the government permission and going on file as the owner of something is pretty standard. Extending that process to yet another thing, even concerning a right that was originally intended as a protection from that government if it became tyrannical, is hardly even considered anymore. Ask permission, pay a fee, and get to have something that one otherwise legally could not? That is hardly different than many Concealed Carry licenses. Asking permission and being on file as someone with permission to use or own a firearm has become pretty routine for many. NFA is the same type of thing, for owning certain types of firearms.
