The wires are not all the same, firstly, pairs are twisted together. Twisting of pairs used for signalling has an important effect on the way external noise is picked up. If you used for a signal pair, two wires that were from different twist-pairs the signal would be degraded.
The colours and pin-ordering follow T568A or T568B as specified in TIA/EIA-568. So far as I know, the same ordering is used for Cat5/5E as for Cat6.
The ordering for 8P8C connectors was initially established for voice communications where each pair carried a separate voice channel. The innermost pair carried one signal, the next wire to the left and right formed the next pair and so on. This makes sense when you remember that there are a variety of modular connectors used for voice comms with different total numbers of pins. In theory you could plug a 6p4c plug into an 8p8c receptacle and have it work (in practice this is not a good idea). Ethernet termination varies from this to keep the pairs closer together but the idea of keeping a pair on the central positions for compatibility with voice applications means that some compromise has to be made.