Why was computer electronic?

Answer 1

Actually many early computers (e.g. the Harvard Mark I) were mechanical or electromechanical, largely because people did not believe that vacuum tubes were reliable enough. However once this was proven false the fact that electronic computers were many orders of magnitude faster than the mechanical or electromechanical computers, electronic computers rapidly replaced other approaches.

During the 1960s there was significant interest in building fluidics computers (i.e. hydraulic logic circuits) because they would not be disturbed by EMP or radioactivity, but this was abandoned with the introduction of microprocessors in the 1970s.