The big one is lack of a magnetosphere. The inability to constantly regenerate an atmosphere, like Venus with volcanic activity (it has very little magnetosphere as well) and constant erosion by Solar winds....and big events like these CME's: https://www.youtube.com/watch?v=sfNZ_Qw5AHs
will make keeping an atmosphere very difficult. If you can't come up with a way to replenish an atmosphere, the rest is academic.
The big one is lack of a magnetosphere. The inability to constantly regenerate an atmosphere, like Venus with volcanic activity (it has very little magnetosphere as well) and constant erosion by Solar winds....and big events like these CME's: https://www.youtube.com/watch?v=sfNZ_Qw5AHs will make keeping an atmosphere very difficult. If you can't come up with a way to replenish an atmosphere, the rest is academic.