Because Slick Willie and the late Lord Benson decided to classify the Street Sweeper and USAS 12 as a destructive device. Lets be specific on the Franchi SPAS. The SPAS 12 is still legal to own, though banned from importation.
http://www.titleii.com/BardwellOLD/cases1.html
# Gilbert Equipment Co., Inc., v. Higgins, 709 F. Supp. 1071 (D. Ala. 1989)
This is a federal district court decision, and paved the way for the backdoor ban on the USAS-12, Striker and Street Sweeper shotguns by endorsing the ATF's "non- sporting" finding as to the USAS-12 (at issue in this case) and the prior non-sporting finding of the Striker-12, that apparently was not appealed to the courts. A gun must be found to be particularly suitable for sporting use, or readily adaptable to that, in order to be lawfully imported after the Gun Control Act of 1968. Prior to 1984 or so that just meant meeting safety standards. After that ATF decided to become arbiters of what sport was, and decided the USAS-12 wasn't suited for hunting, nor trap or skeet, and thus was unsporting. They rescinded their decision that combat shotgun matches were sport, which had allowed the import of the SPAS-12, and decided they were really police training. However they continued to approve permits for the SPAS-12 until this particular case came to trial. The non-sporting finding meant the gun was not covered by the "sporting shotgun" exemption to being considered a DD, although ATF didn't pick up on this for a number of years. In any case the Crime Bill solved the problem, making law out of what had been administrative rulings from ATF. The non-sporting finding is crap made up after the case was filed, and not part of the administrative record. As the court itself notes, one of the principles of administrative law (basically how the courts will treat actions by administrative agencies like ATF) is that the record generated by the process itself must justify the decision, the Agency cannot make stuff up to justify the decision after the fact. Period. But the court allows it here, basically because (I think) they agreed that this gun needed to be banned from import due to its evil looks.
The case also has a rather interesting discussion of whether or not the 2nd amendment includes a right to import guns. The judge (magistrate really) concedes who the right applies to (people, not states), but refuses to agree "keep and bear" includes importation. I don't know where he thought the guns to be borne were going to come from though. And he says flat out, to decide otherwise would gut gun control laws he thinks are good.
maybe if you painted them pink, 
then they would not be so ugly and baned in your state.
