why is charge on body cannot be less than electronis charge?

Well, in classical physics the electronic charge or charge on an electron is considered to be fundamental in nature and thus no charge smaller than it can occur. This is probably due to the fact that during the early to mid twentieth century physicists weren't aware of any smaller charges, they weren't discovered yet. 

However in reality that is not true, as subsequent discoveries suggested, particles known as quarks having charges lesser in magnitude than electronic charge do exist in nature.