Why does the earth's bias force pull the wind at high altitude until the wind direction is 90 degrees with the horizontal pressure gradient force?

Because high-altitude paddling is only affected by two forces, horizontal pressure gradient force and geostrophic deflection force. Only when the two forces reach an equilibrium wind direction can it be stable, and when the wind direction is stable, it happens that the wind direction is parallel to the isobar. However, this is only an ideal situation. The isobar at high altitude is not necessarily parallel, but curved, but in any case, the wind at high altitude is stronger than the wind near the ground.