Popularity, tradition, laziness :)

The danger of electricity is the first thing about it that you learn as a little child. And then you learn: the more voltage, the more danger. Even such people who never learn anything else about electricity know that all their life. Now the voltage has an unbeatable popularity.

Power sources with a fixed voltage were invented before such ones with a fixed amperage. Therefore the voltage was used as “the” number to tell the size of it. People like to be lazy, they like to have one single number to tell the size of things. Accordingly, for the devices that use electricity the traditon was established to say only the voltage (it must fit to the voltage of the source) and then be satisfied.