The define their standard based on the architecture they were developed on. At some point someone ported them to a different architecture and fixed the numerous endian bugs that the original had (because it had never been tested on another architecture).
In otherwords, while you can code up a filesystem for a specific endian format, it is highly unlikely that it will actually be endian-agnostic until someone actually tries to port it to some other architecture, tests against an original image, and fixes the (likely) bugs that crop up. -Matt
