It could be a rotational inertia that we do not know about or understand.
But, inertia doesn't account for the air staying mostly still.
Inertia says the air should fly straight off into space.
Gravity is supposed to pulling those air molecules down enough to stay in orbit around the planet.
However, in this newtonian model, there also exists friction, and that would slow down the air.

If you try to map all the vectors of the air and assume gravity keeps them in orbit all around the planet, then you get weird looking air flows. You see, only at the equator is the air being pulled down into an orbit that would possibly stay on the planet. Towards the poles, the air movement and gravity are not aligned with the latitude, so all the air would be constantly pushed down towards the equator.