The are many reasons for this trend. First, these systems are very affordable, which is important in during this hard economic time. Second, those new, thinly styled LCD and Plasma TVs don't have room for a decent speaker system, so sound bars provide an easy way to get better sound quality for the TV viewing experience. Third, sound bars don't take up a lot of space, and they eliminate the hassle of setting up a lot speakers. Check out some sound bar product options.
However, on the down side, sound bars, can't provide the same type of surround sound listening experience as a multi-speaker 5.1 or 7.1 channel home theater system.
Also, as sound bars continue to proliferate like "bunny rabbits", are they pushing consumers away from a true home theater experience, just as the iPod has pushed many consumers to listen less than stellar quality music on earphones?
For more details about the impact of sound bars in the consumer electronics landscape, read the report from TWICE.