The way I understand it is that Doom was made in an unusually modern and more modular way, you only need to port a few distinct things and then the engine handles the rest. That way of making a game engine is commonplace now, but was unique back then. Combined with the game being low spec by today's standards and a beloved classic, and someone will port it just to do it.
To-Far-Away-Times t1_iugblje wrote
Reply to What makes Doom so “easy” to run on anything? by fox_91
The way I understand it is that Doom was made in an unusually modern and more modular way, you only need to port a few distinct things and then the engine handles the rest. That way of making a game engine is commonplace now, but was unique back then. Combined with the game being low spec by today's standards and a beloved classic, and someone will port it just to do it.