I think the reason for this is probably more: 1. Risk aversion. "Nobody gets fire for buying IBM" still has some truth to it. Even if they don't deliver, the f

I think the reason for this is probably more: 1. Risk aversion. "Nobody gets fire for buying IBM" still has some truth to it. Even if they don't deliver, the fact that IBM is one of (or the?) largest consulting firms in the world is in itself a defense for selecting them. 2. The government bid process. I don't know how this works in the UK, but in the US you have to jump through a lot of hoops if you want to work on government contract, so the same few firms tend to get selected a lot not because they are good, but because they know how to work the process.