Why C# is not fully OOP language? it's one of the critisism.

The difference is that classes are reference types and structs are value types.

A variable of a reference type contains a reference (a type of pointer) to where an actual object of that type is stored on the heap. As well as their fields, objects also need some additional overhead (8 bytes on a 32 bit and 16 bytes on a 64 bit system) for various purposes. When there are no longer any references to an object, it is automatically destroyed and the memory it was using is reclaimed by the garbage collector.

In contrast, a variable of a value type contains an actual instance of the value type, not a pointer to it. No overhead is needed.

Structs are typically used in C# for small (ideally no more than 16 bytes), frequently used, short lived data types. When confined to such types, they are much more efficient than classes since they don't have the overhead of the latter and are not subject to garbage collection. 

It wouldn't be feasible to remove structs from C# since all the built in numeric types (int, double, byte etc) as well as char and bool are simply aliases for structs in the .NET framework. For example int is an alias for the System.Int32 struct.