Any product is imported or exported on the basis of the ability of a marketer and sales force to sell that product in a given market with the assistance of advertising and public relations campaigns.
They all used to be. I can remember when there were dealerships that specialized in a variety of imported cars. There was Austin-Healey, Jensen, Rover, MG, Triumph, Rolls Royce/Bentley, Cooper, Sterling, Lotus, Aston-Martin/Lagonda, Fiat, Alfa Romeo, Lancia, Maserati, Pantera, Lamborghini, Facel-Vega, Renault, Peugeot, Citroen, Opel, and Yugo.
Some of them even tried alliances with American car companies to keep their hand in the American market (Buick-Opel, Chrysler-Maserati, etc). But the bottom line is sales and saleability. If people won't support you by buying your products, you will have to "downsize" your market, and even eventually withdraw from it. As all of these companies did.
British cars had poor electrical systems, often overheated, and leaked oil, French cars were pure crap in terms of serviceability, and the Italian cars were finicky and required too much maintenance (hence the slur for Fiat meaning Fix It Again, Tony). The others simply did not meet the needs of a "maturing" American car market.
If you want to be boggled by the names of cars that were sold in this country, go to:
http://www.collectorcartraderonline.com/adsearch.html
And do a search through the various car brands. You might be amazed...
As for the Citroen, the owner of the one you saw, actually lowered the car via a lever that purges the suspension system of all fluid, and thereby lowers the car. This procedure is not required to park it, just an affectation. I owned two Citroens. They were smooth, comfortable, weird as hell is during winter, and very unreliable.
That's essentially why they all lost their markets in the U.S., and haven't regained them.