Question

why is heat transfer by convection possible only in gases and liquids

Answer 1

It is because convection is the mode of heat transfer in loosely packed molecules.In gases and liquids conduction is not proper because in conduction heat is transferred by one particle to another.As the molecules are loosely packed in liquids and gases conduction will be very hard to take place as the molecules move often.In space as you know there is no medium.The process of conduction and convection is possible only in the presence of a medium.So in space the heat is transferred by the process of Radiation.