Why are acetylene and air commonly used for AAS?
Because it gives a very high temperature: 2200-2400 $^\circ$C. 
This is due to its high heat of combustion, around 1300 kJ/ mol. 

If still higher temperatures are required, as for Al, Si, Ti, etc., a combination of acetylene and $\ce{N2O}$ is used, which reaches 2600-2800 $^\circ$C.