The risk of updating the machines to support IE9 might indeed be large, for not very obvious benefits. But what did they say about staying as is, and switching to Firefox or Chrome? Was it impossible due to use of some MS-only tech?
It's hard to remember the exact details all these years later... I doubt it was due to MS-only tech, but rather that IE6/7 were tested and approved and everything else was not. The incentives for IT teams are such that it's a lot easier to say no to something than yes, and create a ton of work and liability.