Why are computers digital but not an analogue? Claude Shannon has answered this question. Digital signal is easy to amplify and keeping integrity of analogue signal is hard.
You mean probably binary. Binary is easy to understand(on/off), while ternary(developed in the soviet union) were complicate to understand and implement.
MS Press recently released Charles Petzold's 2nd edition of Code. It provides insight into the evolution of computers, including Boolean logic and binary code: http://www.charlespetzold.com/books/
They haven't all been base two (binary.) But the efficiency of designing and building logic circuits for base two won out, soon enough. Building a circuit to say, add or divide numbers in base ten is more awkward, but it has been done. I think base three has been tried, too.