why are electrons negatively charged?

Well, you could give a limited explanation by regarding protons and electrons from the quark level.
On this level, the explanation of the difference in the electron/proton charge (and, for that matter, the neutron's non-charge) is that electrons and protons are constituted from different sets of quarks (i.e. quarks with different charges)
I believe that the proton is constituted by 2 quarks with 2/3 charge units each, along with one quark that has -1/3 charge unit (2/3+2/3-1/3=1).
The electron, I believe, consists of 3 -1/3-quarks (-1/3-1/3-1/3=-1)
The neutron (2/3-1/3-1/3=0) (I think..)

Only thing I can think of to explain it is: that's the way the universe is. The particles have a property that attracts and repels based on something which we have arbitrarily defined as a "charge". The "charge" behaves according to certain rules, and it is best described in terms of positive and negative terms.

 dont thnk theres an answer at this level..

u need 2 get higher knwledge

BEST OF LUCK!!!!!

william crookes studied the electrical conductivity of gases . where he found that a ray  was emitted out from cathode known as cathode rays..

when these rays were subjected to electric field , cathode rays was deflected towards positive plate and .so they were characterised as rays which contain certain negatively charged particles called electrons .

i hope u like the answer ....