Why should Martians be carbon based?
Why can we assume that life on Mars is carbon-based, Phil?
Essentially we assume that because that's all we know. People have speculated that there could be silicon-based life or other, more exotic forms of life but we've only ever seen carbon-based life. It's a good place to start, really. Something that's based on carbon and water is probably what we're looking for. If we look for something more exotic we'd probably struggle to find it in the first place.