Also air traffic control. Taipei Taoyuan airport approach for landing is normally from the northeast to the southwest, given the prevailing winds for much of the year. To get lined up with navigation aids inbound, you have to be out over the sea beyond the northern end of the island. Given domestic air traffic flying at lower levels, plus Taiwan's hilly terrain, it makes the most sense to keep international traffic over water as much as possible. Military considerations might be another issue.