The GTX 1070 uses the Pascal base design, which also features an improved version of NVIDIA’s Maxwell architecture. In contrast, the RTX 2080 boasts the more advanced Turing architecture with RT and Tensor Cores responsible for enhanced ray tracing.
An often underestimated feature is the process size. Any GPU or CPU enthusiast worth their salt will tell you how the process size affects overall performance. The GTX 1070 comes with a 16 nm process size, whereas the RTX 2080 has a 12 nm process size.