If a NAND gate is universal, why you don't have NAND OISCs
With such an instruction set, all you could express are straight-line programs.  Without branches, you can't have loops.  Thus, the program would not be able to handle arbitrary-length inputs: it would be limited to dealing with fixed-size inputs (or inputs with a fixed known upper bound on the size of the input).  So, that wouldn't be very satisfactory.