Most operating systems are written in Assembler, for a specific architecture. It is therefore VERY painful - if at all possible - to `port' the operating system to other architectures.
UNIX, on the other hand, is mostly written in the C language. This alone allows UNIX to be portable to many architectures. Today, UNIX runs on more architectures than any other operating system in the world. Examples of such architectures/processors are the Motorola 680X0-based workstations, the 80X86 machines, the RISC based architectures (SPARC, MIPS, 88000), VAXes, IBM mainframes, Amdahl, Cray, and many more.
Table 1.1 lists the different UNIX versions vendors use.
Table 1.1: UNIX variants on the market.