Why is the ABI for structs different between Linux and Windows?

[Don]

Walter Bright wrote:
> The struct ABI is designed to be compatible with the C struct behavior 
> for the local C compiler.

That's clear enough for extern(C) ... but why does it need to be that 
way for extern(D) ? (or internal D functions, for that matter).

Why not take the opportunity to standardize the D calling convention for 
a single CPU?