Probably because IBM isn't really in the consumer market, they focus on research and enterprise so they don't have as much skin in the game.

This isn't about what's right or wrong, Google, MSFT, and Facebook have the most to lose if the USG can compel them to intentionally reverse any security improvements they implement both in terms of operational overhead costs and in actual loss of customers as they would be more likely to migrate to other services that are either non-US based or those who haven't been squeezed yet by the USG.

This is going to be a game of domino's the 1st one to fall will set a precedent that then could be easily used in any court and if Apple falls, MSFT, Google and Facebook will quite likely be affected while IBM probably wont.