It costs a lot of time and energy to shape, maintain, and then move around a digital signal. Speed, as it turns out, is not everything. Errors and inaccuracies, for example, can be controlled within a digital computer much more precisely than in an analog computer.
Digital computers are much easier to program too.
There is one place, however, where heat must be produced. Whenever information is erased the phase space associated with the system that stores that information must shrink. Processing information occurs by the means of manipulating the system on which the information has been written. Erasing a single bit of information reduces entropy of the system that stored that information by at least . This reduction of entropy results in heat transfer to the environment.