why we can not use <iostream.h>

Answer 1

You can use iostream.h .The reason why it is not set as default is due to the fact that iostream.h is mainly set for C. iostream is set for C++.Although it is possible to use iostream.h on C++ ，it is slightly more limited and may cause bugs.

Answer 2

You can use it.

#include <iostream>
using namespace std;

int main()
{
cout << "Hello!";
}