Question should be how not why ? :-)

As you say  Asad ...how ? 

In true sense, GO does not form true solution. Instead, it forms a  relatively stable homogeneous dispersion in polar solvents like water/ alcohol on prolonged sonication. Obviously, the dispersibility will be dependent on the aspect ratio of the GO sheets. Hence, it is quite natural that if your sheets are larger then it will not get solubilized in EtOH. May be it has good proportions of polar groups leading to its high dispersing ability in EtOH