Additives (engine oil has more) and weight (gear oil is heavier).
Now, some modern manual transmissions use ATF (Automatic Transmission Fluid) in them.
Different viscosity. In very simplistic terms an Engine requires a thinner oil than a gearbox because the components have different tolerances and speeds and different oiling mechanisms. A engine uses a pump to keep a constant flowl. Most manual gearboxes rely on a sump and essentially a oil bath. Thicker oil clings long enough to get pulled to the top of the gear box and coat the rest of the gears as it drains back down to the sump, much the same as rear end gears are also a oil bath in your old Ford 9" or Chevy 12 bolt posi.
MTF (Manual Transmission Fluid) is in fact a 5w-30 engine oil.
There are cars where the gearbox runs on the engine oil, old minis for example.
For your information a 75w-90 gear oil is roughly equivalent in viscosity terms to a 10w-40 engine oil.
Sulfur is what makes gear oils smell so bad.
Sulfur compounds are part of the extreme pressure lubrication additive pack needed by hypoid gears. Hypoid gears are found in rear-drive differential ring-and-pinion sets where the centerline of the pinion is offset below the centerline of the ring gear- it allows for the driveshaft to be lower and the floor of the car to be flatter. The gears themselves therefore are sort of halfway between normal straight-cut bevel gears and a worm gear. That means that as the teeth engage and release, they brush across each other while under high pressure. Special additives are needed to keep the specially hardened surfaces from degrading under that kind of contact. Its a bit like flat tappet cams where you have a sliding motion and high pressure at the same time, although not quite as extreme as the tappets.
