The reasons were given. It was something along the lines of it being too important to be left to the industry alone to decide, and that the SRA/DfT saw it as 'their job'.
Seems a bit weird to me. They trusted Virgin with ordering Pendolinos and Voyagers yet cannot trust the others to order some themselves. But the SRA is odd in its decisions...
For the worst company at ordering new trains I vote the DfT, they are great at messing up the specification. GWR has bad seating and a lack of buffet yet for some reason there is a kitchen on all sets. They also have far too many 5 car sets, which require more crew. Its not uncommon for a 5 car set to show up instead of 2x5car leading to bad overcrowding and often people unable to board the train. LNER/VTEC got a good deal, with mostly 9 car sets and a buffet, while the seats aren't different the seat covers seem to improve it for most people. GWR's 5 car sets are apparently due to a timetable planned by the DfT which had a lot of splitting and joining of trains to serve more destinations, they never bothered to check this timetable over with FGW/GWR until it was too late and they had made the order.
Meanwhile the 700s were a good choice but have far too bad seats, I understand they need a lot of standing space but surely they could give the seats a bit more cushioning to make them less hard, although apparently they approved with age, and I think a moquette instead of the bland blue covers would help people's thoughts on them. They also didn't specify WiFi, even when Siemens asked them if they wanted the wiring put to future proof it they said no, now WiFi has been retrofitted at a higher cost because the DfT didn't get the wiring put in when the trains were built. Similar situation happened with seat back tables, they weren't originally specified, now getting put in at high cost.
On a side note does anyone know why the 800s have an odd engine layout? They have 3 940hp (they got uprated later on from the original 750hp) engines per 5 car and 5 940hp per 9 car. Having 1 523hp engine (As used in the CAF Civity) per car would give a similar amount of power while I imagine being easier to fit in.