The simple answer is that software needs to be build for a specific hardware platform. So Linux IA32 is specifically build for the IA32 architecture. You cannot run Linux IA32 on a MIPS platform for example. Then there is binary compatible and software compatible.

Binary compatible means you can run the pre-build software (the binaries) and run them on another platform. Software build for I386 can run on AMD64 for example.

Software compatible means you can use the exact same source but you will need to recompile for a perticular platform. I cannot remember an example for this