Why can't enum types be underlying types that are integral types in the System namespace?

Question

I've done quite a bit of PInvoke stuff recently, wanting to stay close to the Windows API header files.

When creating the enum below, you get this compiler error:

Error   1   Type byte, sbyte, short, ushort, int, uint, long, or ulong expected 

Why is it that the C# compiler does not understand any other underlying types?
The C# compiler can go from uint to System.UInt32, why not the other way around?

using System;
using System.Runtime.InteropServices;

using DWORD = System.UInt32;

namespace BeSharp.Win32
{
    [StructLayout(LayoutKind.Sequential)]
    internal class MOUSEINPUT
    {
        // ...
        MouseData mouseData; // DWORD
        // ...

        [Flags]
        internal enum MouseData : System.UInt32; // needs to be uint; you cannot do DWORD or System.UInt32 here
        {
            Nothing = 0x0000,
            XBUTTON1 = 0x0001,
            XBUTTON2 = 0x0002,
        }
        // ...
    }
}