IMO the real reason is summed up in the last paragraph.
"The best-known assault weapons are the AR-15 rifle and its many spin-offs. While rates of gun ownership have fallen over the past four decades — about 50 percent of households owned a gun in the 1970s, compared with 31 percent in 2014 — sales of assault weapons soared after a federal ban expired in 2004. These rifles are now "the bread and butter" of the gun industry, says Josh Sugarmann, executive director of the Violence Policy Center, which advocates gun control measures...
Rifles accounted for just 3 percent of gun homicides in 2014, and 27 percent of public mass shootings — single incidents in which four or more people are shot dead — from 1999 to 2013. But in recent years, there is growing evidence that the assault weapon has become the mass shooter's gun of choice...
Many gun owners see the firepower that assault weapons provide as a comforting form of insurance against government tyranny, crime, and terrorism. "This firearm gives average people the advantage they so desperately need and deserve to protect their life, liberty, and happiness," says gun-rights activist Dom Raso...
The 'Barack Boom'
Sales of assault weapons have grown by about 27 percent a year since 2007 — and much of that demand has been driven by the election and presidency of Barack Obama. The "Barack Boom" has given the gun industry a $9 billion boost, according to
The Washington Post, thanks largely to the National Rifle Association's insistence that Obama is plotting to take away all guns.
http://theweek.com/articles/634386/why-are-assault-weapons-popular