A few reasons.
Prominent among them is that $200 used to be significant. A $200 tax was close to the new cost of a Thompson. A nice pre-war Thompson was quite an expensive firearm at the time. These were the ones typically seen with barrel fins. By 1928 it was around $225.
By comparison an M3 grease gun price around WW2, over a decade later, was about $20.
So the tax stamp alone cost 10x what the inexpensive full auto of the time cost even after 10 years of inflation.
Compare that to the cost of a new MAC-10 the more modern inexpensive firearm before the close of the registry. Around 1980 the price of a brand new full auto Mac-10 was around $200-$250. (Consider a semi-auto version is about a $500 gun today, about the same price of the full auto version without the artificially restricted market.)
So the tax stamp still doubled the price of such a firearm in 1980.
A second major reason is many people are untrusting of government registration, even more so in previous times. Registration around the world has often preceded confiscation. Or results in easy confiscation when the new legislation further restricts previously registered firearms in ways many owners are unable to comply with.
Registration of standard firearms in most of the country was quite rare and is still rare, but was even more rare before the close of the machinegun registry in 1986. Standard firearms were sold no differently than tools at your hardware store in many places until 1968. Mail order catalogs were common. You could mail order surplus Lahti 20mm rifles for little more than the cost of a standard hunting rifle (before 1968 when the 1934 NFA was amended with the Destructive Device category.)
Except for the NFA. The NFA was one of the few registries where you could sign up as a known threat with the government.
Today people have become more familiar with signing up with the government as a firearm owner, in many thanks to CCW, where signing up to carry a gun is standard.
People have become more accustomed to formalities and forms with firearms today than they used to be. What many would have been uncomfortable doing before the 1980s they will readily do today.
Which is another reason far more people readily purchase other NFA items than in years past.
A third reason was many CLEO would not sign off on a machinegun purchase. You couldn't just look up a way around that on the internet back then, and that stopped many people cold. Chief law enforcement officer of your area didn't want you having a machinegun and you didn't get to have one.
Also consider that the NFA was for all intents originally meant as a ban. Back in a time when Congress believed it had no legal right under the Constitution to ban anything under the commerce clause, only to regulate trade.
So they banned by placing a tax almost nobody would pay, and most wouldn't even know about.
Consider for example they also banned a drug, Marijuana, the same way. With the Marihuana Tax Act, requiring essentially the same thing as required by the NFA. (Marijuana was legal up until almost WW2.)
The purpose of either tax stamp requirement was not to simply have people paying a fee and continuing to trade in the product Congress intentionally targeted.
No the purpose of the stamps was to ride society of legal trade all together, and provide a means to arrest people and put them into prison for possessing or trading the targeted product.
So consider that. It was not just a formal process you went through as it is viewed today, it was a ban at a time government could not simply ban outright, and carried a criminal stigma of suspicion for a regular person to even try to go through the process.
Signing up as a registered machinegun owner was akin to signing up as a criminal to keep an eye on with the federal government by going through a process they didn't want normal people to go through.
That would change over time, and an increasing number of regular people would go go through the NFA process. But when originally enacted and for some years afterwards you would be highly suspicious and akin to a bad guy by trying to have such a firearm.
:banghead: