I always thought it was because both languages were (at least at some point) referred to and advertised as 'systems languages', which naturally invited comparisons. However, 'systems' meant different things for each language.
For Go, 'systems' as in the sense of software running on servers providing services to user applications.
For Rust, 'systems' as in the sense of (http://en.wikipedia.org/wiki/System_programming_language) - software such as OSes, device drivers, etc.
Just for fun, Swift is advertised as a systems language as well. In its defense, it does have facilities for bypassing the preferred way of managing memory (ARC) and working with memory directly.