The reason is the width of the soundbar itself. Amazon dimensions put it at about 38" wide, and a typical 40" HDTV is about 36" wide depending on bezel thickness. The soundbar will definitely work for smaller TV screens, but it will just look funny since it will be wider than the TV screen itself. The soundbar looks perfect beneath my 50" Samsung (about 45" wide).
Exact same thing happened to me. I am in the process of shipping it out for repair. The unit is of terrible construction and the electronics flaked. I actually got a sub -woofer and wireless surround speakers for my TV which are 10 times better.
Does this mean you already have a surround sound speaker system or only the receiver? As an add on, meaning using it as a center channel, might be OK. I found the mid range lacking. The sound bar comes with a subwoofer. Would you be duplicating what you already have? The connections go from the receiver to the subwoofer then to the sound bar. Does this help?