The law says that it is illegal to import a 3 wheeled (1 wheel in front, 2 in back) ATV for offroad use into the good old freedom loving United States of America, but that wasn't added until early 2000's. 1987 is the year that manufacturers voluntarily ceased production for most models, a couple have extended into 1988. There was too much bad rep (thanks BBC) which swayed public opinion. Nothing was factually proven to be faulty in the design leading to instability, all it was is media hype and faking stats/demonstrations to prove an agenda. Most think it was a certain manufacturer which now sells bikes with a red "S" on them that paid off media organizations to sway public opinion to not want to buy the vehicles because said company did very poorly with R&D and actual sales. By 1986, 3 wheeler sales had diminished drastically enough that it wasn't viable to keep producing them. Most of the remaining new stock of 3 wheelers were either destroyed or donated to schools for workshop classes, some were even converted to 4 wheelers and sold as new.
The law does not say it is illegal to modify existing machines into 3 wheeled designs, companies like Tionista Trikes or TPC Trikes have taken TRX450R quads and CRF450R bikes and spliced the frames together to make a 3 wheeled design, complete with custom machined triple trees. Makes for a very modern looking trike that is nothing but gnarly down to the bone:
Manufacturers can still produce conventional or reverse 3 wheeled designs for the road, just not for offroad. If you ask me, it's a stupid and pointless law that makes zero sense. IF it were gone, manufacturers might take up an interest in producing a 3 wheeler again, considering the popularity of the machines has been increasing to levels of popularity not seen since the 80's. SCORE has been working to add a vintage 3 wheeler class to their Baja races, most major ATV tracks now let the 3 wheelers race again, and they have been receiving a lot of positive publicity from major magazines.