Because it's likely that it wouldn't end with "unlock this 5C" -- it would eventually extend to the government forcing Apple to either stop providing the additional security features in its newer models, or find ways to ship something that looks kinda like the security feature but isn't really.

Drawing the line in the sand at "the government can't force us to hack this guy's phone this time" thus ends up being "can't force us to provide features to hack anyone else's phone down the line".