Qubit are generally an abstraction of a 2 level system. @lemouth is right when saying that you could think of exploring multi level systems (and some researchers have), but most current quantum computing research focuses on 2 level systems. Most importantly, the gains in computing comes from the superposition of states. This is often explain as having an infinite number of state instead of simply 0 or 1, and although that is not incorrect, it is in fact fundamentally much more then just a continuum of states (otherwise you could do the same with an analog computer where you have infinite voltages between 0 and 1).