Firstly, according to the latest accident statistics of transportation in Thailand, the bus accidents are found ten times of the airplane ones apart from the fewer amount of insurance payback in case of decease.
Secondly, in rush hours or other case of emergency, the passengers tend to look for the best choice helping them reach their destination faster. For example, Bangkok - Chiang Mai trip can be made shorter by plane within 1 hrs10 minutes. In contrast, bus taker at least 9 hours. The 8 times of difference in time consuming are obviously accepted.