It's open-source hardware. Anyone can make/modify/sell the design. The restrictions are duplicating trademarked labels that the "official" boards carry.
If I had to guess, the Nano, Micro, and Pro Mini inherited that pin order from the pin choice made on the Mini.
Why did the Mini make that choice? I don't really know, but my guess is it made layout to the serial header (which duplicated TX/RX pins) easier.