Why can't a const integer be used as the size of an array?

Question

#include <stdio.h>

const int i=1;
int a[i];

int main()
{
    printf("Hello World");

    return 0;
}

Size of an array must be a constant expression. I have declared i to be const, why still the error?

main.c:12:5: error: variably modified 'a' at file scope
 int a[i];
     ^

Why does moving the part inside the function main work:

#include <stdio.h>

int main()
{
    const int i=1;
    int a[i];
    printf("Hello World");

    return 0;
}

Thanks.