To be strictly accurate it is 'BT Openreach'. As to why they alone are doing it, you have a lot of history to catch up on and you will then see how they were the only bidder (conveniently?) left in the frame.
It is quite concerning how little the 'man in the street' actually knows of the process that has unfolded and how his expectations of 80mb are actually going to be a minimum of 'feeling like' 15mb (unless he is 'missed out' in which case he will get >=2).