I think the BattleTech series had a really good in universe explanation.
Just like in real life, heat == bad for machinery. Cooks wires, expands joint seizing them up, softens metals, desolders circuitry.
In a real-time strategy game, abstracting overheating to HP is a good design concession for simplicity's sake.
What makes organic life forms less fire resistant is The Dinner Effect. Even the outermost layers of organic things is made of Dinner. No robots are made of Dinner and therefore not so flammable.
A case could be made that flamethrower units could be resistant to fire, however. But I'm not a big fan of BuT sOmEtImEs mechanics. It would need to be a more broadly applied rule rather than weird exception.