It's not about killing people, an attack on the airport is a higher profile attack regardless of how many casualties it involved.

Attack on the airport also targets people who aren't native, often westerners which is both what these organizations often want and it ensures that it would much more widely reported than "a bus blew up in north eastern turkey" which maybe will get a headline for 2 hours.

Attack on an airport tells people see you aren't safe anywhere your "most secure" location with all that security isn't safe, we can get you anywhere any time, this helps to spread the fear both within the country and world wide.

Attack on an airport immediately triggers security lockdowns world wide which amplifies the financial cost of the attack globally.

Terrorism is often considerably more calculated than what people think, especially when it comes to high profile attacks.

Putting 10 bombs on 10 buses requires a lot more planning and terrorists, which greatly enhances the chance of getting caught. Especially in the US, where much of the risk is currently centered around "lone wolf"-type attacks, anything that would require a greater number of people seems like it would be a significant deterrent.

I don't follow. Overnight bus parking lots tend to have very little security. Luggage loaded onto buses gets very little screening. A single person could easily put bombs on multiple buses and then slip away.

Or ten people could do it. It's probably a lot easier to find ten recruits willing to load bombs onto buses and escape than to find one recruit willing to die by bringing a bomb onto a plane.

If you go around trying to find 10 people to commit mass murder, there is a high likelihood that one of them will turn you in, or that one of them is already under law enforcement scrutiny that will lead to discovery of your plans.

Even easier to find one person willing to give ten recruits a carefully wrapped package of "drugs" that they think they will deliver at the other end....