Mostly its a "no one gets fired for choosing IBM" thing.

Imagine the position of the CTO if the project went to a smaller sized, not so well known, consulting firm, and it failed. The CTO would be responsible for the failure.

But with firms like Accenture, it would be the firms fault as they "are the experts".

Sadly, IT knowledge is low enough in the general population that this shifting of blame will be the norm for the foreseeable future.