The special treatment broadly predates CIDR and official documentation of general arbitrary-prefix-length subnetting! In terms of why there were two broadcast

The special treatment broadly predates CIDR and official documentation of general arbitrary-prefix-length subnetting! In terms of why there were two broadcast addresses, this apparently has to do with late standardization plus inadequate communication between different groups of TCP/IP implementers (Berkeley and non-Berkeley), plus an attempt to maintain backwards compatibility with 4.2BSD that was never revisited. Seriously, the rationale in all the RFCs that mention the lowest address being special is ultimately compatibility with 4.2BSD. As I mentioned elsewhere in this thread, several OSes no longer enforce this special case. But people on forums are often willing to confabulate justifications for why the special case should have existed, such as the idea that it would be impossible to distinguish the network-as-a-whole from the host at that address. (I guess those people must also believe that array indices have to start at 1, because otherwise it would be impossible to distinguish an array from its first element?)