This is how bans and grandfather clauses are effective. The market for machine guns once existed in the 70's and early 80's, it was a hobby enjoyed by those that jumped through NFA hoops that many others considered an infringement of thier rights (this is a time when computer databases were few, and research required looking through filed paperwork slowly and tediously). Keep in mind the NFA was supposed to be a ban for everyone except the rich in 1934. $200 back around the time of Prohibition and the Depression was more than anyone except today's equivalent of millionaires could afford. Here is a price list in 1934:
How Much things cost in 1934 Average Cost of
new house $5,970.00
Average wages per year $1,600.00
Cost of a gallon of Gas 10 cents
Average Cost for house rent $20.00 per month
A loaf of Bread 8 cents
A LB of Hamburger Meat 12 cents
Arrow Mens Shirt $2.50
Studebaker Truck $625.00
The average income in 2006 was $34,892 for the entire US according to
http://data.bls.gov/cgi-bin/surveymost.
$200 was 12.5% of a person's yearly income. That would be the same as a stamp price of $4361.50 in 2006. That is on top of the price of the gun. This was during a time when most people were struggling to survive, nevermined afford extras.
How many people would purchase a firearm today, and then pay a tax of $4400? Probably only a select few right? Now imagine that was attached to an item seen in those days as a cheap affordable tool and not a hobby like today. Would you buy a $5 Hammer if it had a $200 tax when you could use another item as a hammer instead?
Yet due to inflation that stamp price gradualy fell into the price range of more and more people. To the tune that by the early 80's the average man that was willing to register with the federal government and was aware of the NFA hoops (no public internet back then) they could jump through could afford to own a fully automatic firearm. This was simply not acceptable, and since government was felt to have more authority in 1986 than anyone believed in 1934 a more serious ban could be implemented. Yet a clause was added to let those who already owned them continue to do so to avoid organized political opposition. Effectively silencing those that would care.
So you see the average man was never meant to be able to afford the hobby of full auto arms. Only inflation allowed it. By limiting it to only the very wealthy people they are kept out of the hands of those likely to use them for personal gain in crimes(in theory since they won't be desperate enough, same reason 'saturday night specials' and other "too affordable" guns not used by LE are banned). The legal number is also so low that any illegal use of them in theory is easy to track to the owner.
However with drug cartels, and easy modifications, as well as numerous parts of the world with easy full auto markets, criminals that wish to are able to use full auto arms anyways. However since most crimes involve something a criminal can conceal, whether it be a gun or a knife etc..and there being few full auto handguns, or semi auto versions of full auto ones(and therefore easily converted like the infamous KG-9, or early Tec 9), they are rarely used by the average street criminal. Thier use by organized crime though in planned crimes like along the border persists, but that is more because guns are outlawed altogether in Mexico, and so full auto and semi auto use in crimes makes little difference.. However even organized crime avoids items that will bring more investigative manpower down on them in the USA. They want to make things appear to be "normal" and not unique.
So you see it is really a class issue, and only those with a lot of money and successful financialy, or politicly important, are deemed suitable for full auto weaponry. Since the value of the legal full auto weaponry left is rising faster than inflation, and since existing guns are lost or destroyed this situation will not change. So no new laws governing them is seen as necessary. Eventualy most states and locations will have banned thier existance anyways with laws intended for semi auto firearms, like assault weapon laws which they fall under.
However when gun ownership is effectively banned altogether you will likely see a dramatic increase in thier use since those that still own or use firearms will not be trying to work around ownership rules, and just getting the most effective tool for the purpose they want it for.
I can't speak for the NRA, but I know very few NFA owners who feel that way. (what do I know though, I'm just an NFA dealer) 
