because it is a class that is specifically designed to be only derived from.that class has pure virtual functions that must be overriden by the class inherits abstract class.instantiating a class that has pure virtual functions is pointless and might cause compiler errors i guess.for example think of a Bird abstract class that has many pure virtual functions like fly() that have to be defined by the inherting bird kind lets say Sparrow.If you could instantinate Bird class lets say Bird mybird then mybird::fly() and fly function is undefined that would not be logical.besides can you show me a bird that is only a bird? no you can not do that.so instantiating a bird class is not logical.i am not an expert but these are the reasons i can think of.