Why 8085 microprocesseor cannot have more than 256 instruction?

Intel designed the 8085 as a 8 bit computer, with the opcode byte as the first byte of the instruction. As such, there are only 256 possible values for the first opcode byte. (28 = 256) As implemented, Intel provided only 83 different instructions.

It is possible, however, in general, that secondary opcode space be usable, so the "8 bit" rule might not necessarily apply, but Intel chose to not implement any of them on the 8085, like they did on the 8086/8088.