Question

why an electron is negatively charged

Answer 1

Because they attracts the positive charged
(proton) ....

Answer 2

ANSWER--

Actually the negative positive charge is just a globally accepted convention that denotes the opposite nature of charge on electron to that of proton. As the charge which an electron will attract is repelled by a proton. Thus it is purely a convention in reality electron does not has a negative charge as that negative sign just denotes the nature of charge.