For the same reason so many other things are not strictly specified by the standard: To allow flexibility to produce a compliant compiler for a large number of platforms and systems, and still have an EFFICIENT compiler.
In particular, bitfields having to be stored in a particular bit/byte-order would make it horribly slow on machines whose natural byte-order is the "wrong way around".
Yes, it means that it's a right pain in the behind to make bitfields portable across multiple archiectures and platforms. If you really need that, then perhaps you should consider some other solution...