Striking designs, such as this 2014 Kia Optima, helped make Hyundai and Kia a compelling choice for many U.S. consumers. But now, sales are falling. Photo credit: Kia Motors

Korean automaker Hyundai (OTC:HYMTF) and its corporate cousin Kia Motors (NASDAQOTH: KIMTF) have had a great run in the U.S. over the last several years. Their combined sales nearly doubled between 2008 and 2012. Maybe more importantly, consumers' perceptions of the brands seemed to have shifted dramatically -- for the better.

Once, Korean cars were seen as low-cost alternatives ... and that's about it. But a new emphasis on design and quality elevated the two brands to near-peer status with Japanese brands like Honda (NYSE:HMC). And in some comparisons, Hyundai actually had Honda beat.

However, things have shifted in the last year, and sales are down. As Motley Fool contributor John Rosevear explains in this video, it's not so much that Hyundai and Kia have changed for the worse -- but rather, that the competition has made it a lot harder for the two to shine.