Trump is ostensibly using the 9/11 play card to justify banning few muslim countries [0].
This is obviously a lie, since most of 911 hijackers were Saudis and his executive order is not banning the Saudis. I'm guessing on the strength of the Saudi lobby in DC [1] AND/OR Trump's potential business conflict in Saudi Arabia [2].
[0] https://www.scribd.com/document/337545704/Draft-Executive-Or...
[1] https://en.wikipedia.org/wiki/Saudi_Arabia_lobby_in_the_Unit...
[2] http://www.theatlantic.com/business/archive/2017/01/donald-t...