Not being a member of WG14, I can't say anything definitive, but I have my own ideas:
It would violate the principle of least surprise - there may be a damned good reason why I want to lay my elements out in a specific order, regardless of whether or not it's the most space-efficient, and I would not want the compiler to rearrange those elements;
It has the potential to break a non-trivial amount of existing code - there's a lot of legacy code out there that relies on things like the address of the struct being the same as the address of the first member (saw a lot of classic MacOS code that made that assumption);
The C99 Rationale directly addresses the second point ("Existing code is important, existing implementations are not") and indirectly addresses the first ("Trust the programmer").
