>>13248824because two of it's moment of inertia's principle axes are of equal length. tumbling requires three different lengths of principle axes, and then one must try to spin the object about the axis with the intermediate length
this is easily demonstrated with a rectangular prism, like a book held shut by rubber bands. try tossing it in the air and spinning it along it's three principle axes. two of them will result in stable rotation, while the third will cause tumbling
