Why is it difficult to write portable C programs?

Question

I want to know, why is it too hard to make a program run in many OSes, like Windows and Linux, without including glue code. They all share the same architecture (x86), so I

Answer 1

C does give more power compared to other higher level languages.

Different Processor (CISC and RISC based), 32 and 64bits does put the bit streams of data in different order. There are some supporting APIs to do the conversion, but not totally reliable for all cases.

Windows and Unix have custom flags which need to be used for building.