It's simple. BAA are more interested in building shops and cafes in the airport than putting money into anything that actually helps get a/c off the ground. EDI is a total joke for an airport that busy, they need full length taxiways now and a control tower that allows the ATC guys to see both ends of the runway.
I believe a little while ago the major airline customers got together and told BAA at EDI that they were now going to backtrack every departure and use full length for every landing rather than trying to make the turn-offs. Howls of protest from BAA about flow rate etc. Well build a proper taxiway was the response from the airlines!. The sooner the better.