Because it isn't politically expedient for them to do so, as Apple is very very successful, and as such, most people (voters in this case) who are uninformed love the theater and cheap political pot shots lobbed at Apple. It's clear that there is no discussion at a national nor international level of the actual implications of what this means for not just Apple, but indeed for the American economy and software built in the US.
For instance, do you really think Microsoft will be able to sell Office software to the Netherlands Government if the DOJ/NSA/whoever can use the All-Writs Act to force Microsoft to implement a backdoor into their software? Would the NSA be able to use the AWA in conjunction with a NSL and a secret court to force the hand of companies? Politicians / the public don't really grasp what's at stake here. What we're really talking about creating a complete and real artificial handicap for all software companies located/based out of the US. Already there is pushback in China, Europe and Australia to ditch American-made software after the Snowden revelations. A ruling in favor of the FBI will only compound and accelerate this issue and will have a marked and measurable effect on the revenues of software and hardware companies located in the US.
While Google/Apple/Facebook/Microsoft/Cisco et al may not be able to relocate, this will definitely cause small and medium sized firms to relocate or possibly to never incorporate within the US to begin with. This may be effectively and preemptively scaring away the next Google. Law of unintended consequences and all that.
Come to think of it, why aren't free market standard bearers rallying against this as government intrusion into market features?