DOS may be easy to emulate and re-implement because it's a single task operating system that does not do much. Most of hardware is accessed directly, and needs to be emulated instead. We enjoy great compatibility because of the enormous leap in performance since then (the slower the system the easier it is to simulate correctly on a modern one), and the combined knowledge of all the ins and outs collected during the PC boom by software authors and hardware makers implementing and re-implementing compatible devices.