There's a bit more variety in the SS acoustic market. You occasionally see 'all-rosewood', for example.
The answer to the original question is no, there is no limit to the choice of top material, in theory. Think resonator guitars or carbon-fibre materials.
More mundanely, spruce and cedar have a long history of use, they are, relatively speaking, easy to source and reasonably priced, and they do the job that is expected. Over time, the sound that is produced when these materials are used has become a 'benchmark' for how a CG 'should' sound. (This is one of the reasons why innovation can be difficult).
I would hazard to suggest that someone could come up with a cheaper, more resonant, more easily prepared, more sustainable, 'nice sounding' material for guitar tops and there would still (and always) be those who will argue that 'it isn't the same' or that 'the old way is better' - which is a valid point, but in the end, a matter of opinion and taste, to some extent.
When someone with major influence comes out and plays a really great sounding CG made of something 'new', then there may start to be change. As of this moment I am not aware of such a material.