energy gained by electron (eV) = accelerating voltage (V)
energy = charge times voltage (aka electric potential)
charge of an electron is e
voltage is V
so the energ yheld by an electron at potential V is eV.
1 eV of kinetic energy is E = VQ = 1 x e joules , because the electron is accelerated by 1 volt and the electron has a charge e, but because this energy e, is small, the electron volt was made as a new unit for convenience, which just means 1 eV is e joules
One electron volt is the amount of energy gained or lost by one electron moving through a pd of 1 volt.
It's rather like asking why a force of 1N accelerates a mass of 1kg with an acceleration of 1m/s/s