Why implicit cast from strongly typed enum to its underlying type is not allowed?

Let's consider the following code:

#include <type_traits>

enum class foo_bar : unsigned
{
    foo,
    bar,
};

int main()
{
    foo_bar bar = foo_bar::bar;
    // unsigned a = bar; -- error: cannot convert ‘foo_bar’ to ‘unsigned int’ in initialization
    unsigned a = static_cast<std::underlying_type<foo_bar>::type>(bar);
    return 0;
}

Why the implicit cast is not allowed? The underlying type is known, type of a matches the foo_bar underlying type. The implicit cast seems to be safe and could be performed without loosing an information. Why the cast is necessary from the language design perspective?