They have ironclad contracts with the DoD and other government organizations, and with the way those processes work if every person from the president to the peon decided they wanted to move everything off of IBM and were going to move heaven and earth to do it. They wouldn't even be able to start entertaining bids from other competitors until 2035.

Beyond this IBM's core competency is winning government contracts, a niche very few other people have the desire or the resources, or the proper understanding of the necronomicon to enter.