It might not be quite so clear.  Boeing's case against Bombardier is extremely weak, they don't have much of a leg to stand on and they know it.  They might have a stronger case if they were arguing that Bombardier received subsidies, but Boeing receives a lot of subsidies too so they probably don't want to open that can of worms.

From an outsiders perspective it seems that the primary goal of Boeing is not to get an injunction against Bombardier (they don't offer a comparable plane so all an injunction would do is shift sales from Bombardier to Embraer), but rather to extract more subsidies from the U.S. government.