For FTTP then I think it's a marketing thing, for ADSL it is because of the technology. There are a limited number of RF channels I believe. You could allocate them such that it was symmetrical but because users generally download they gave more channels to the download side of things.
BT could have rolled out a synchronous down/up speed product at the same speeds as the download speeds of ADSL instead of ADSL - but it would have been more costly for the consumer compared to most consumer needs and as Chris said eat into the leased/business market.
Well you could buy it, it was called SDSL.
Don't think they ever made a SDSL product over 2MBit/s though.