I was thinking that why are their only two bits,0&1. I know that they represent a particular switch in IC being opened or closed. But why not for switch neither completely open nor closed i.e. in intermediate stage relaying only a partial amount of voltage. Then we can have more than one bits, which can make coding much easier.
A bit is, by definition, a binary digit, and binary means base 2.
Base two is the most common model of computing, but it is not the only one. There is also analog computing, which is not that interesting to computer science. Ternary logic, on the other hand, does lead to some interesting theory. In that case, however, each digit would be called a trit.
One advantage of representing numbers in base 3 is that it is more efficient, since 3 is closer to e ≈ 2.71828…. On the other hand, ternary logic is harder to implement reliably in electronic circuits, so binary computing receives nearly all of the attention.