Trump is attempting to prevent the next terrorist threat. Not stop ones that occurred in the 90s. To use a hockey metaphor, you need to skate to where the puck is going to be, not where it used to be.
The nation states that Trump temporarily banned either are in the midst of a refugee crisis with infiltration of radicals amongst them or are failed states which has caused practical issues with vetting, Somalia, Yemen, Libya, Syria, Sudan, have imported to the US in recent years terrorists or terrorist supporters, Iraq, Syria, Somalia, or are on the list of states that officially sponsor terrorism, Sudan, Iran, and Syria, or a combination.
Note that Saudi Arabia doesn't fall into these categories. Egypt either. And while these are observations, I think they are fairly compelling enough to point to a rationale why these countries were banned and not others.
Here is Trump's statement in full, and I think the reasoning above and evidence below supports it:
I'm establishing new vetting measures to keep radical Islamic terrorists out of the United States of America. We don't want them here. We want to ensure that we are not admitting into our country the very threats our soldiers are fighting overseas. We only want to admit those into our country who will support our country and love deeply our people. We will never forget the lessons of 9/11 nor the heroes who lost at the Pentagon. They were the best of us. We will honor them not only with our words, but with our actions, and that's what we're doing today.
The executive action reads:
Deteriorating conditions in certain countries due to war, strife, disaster, and civil unrest increase the likelihood that terrorists will use any means possible to enter our country
Minnesota is the new home for most Somali immigrants. And there is a problem with them some of them joining and abetting ISIS.
The case lay at the intersection of immigration, Islam, and terrorism and, coincidentally, ended the week following the victory of President-elect Donald Trump. To borrow a Trumpian term, the “Minnesota men," as media generically referred to a circle of Somali-American ISIS supporters, are bad hombres. At a campaign stop in Minneapolis on November 6, Trump delivered the message that we "have seen firsthand the problems with faulty refugee vetting, with large numbers of Somali refugees coming into your state, without your knowledge, without your support or approval." It was the sons of some who sought refuge in the United States from the bloodshed in their native land who became enamored of the idea of causing more of it.
The first "Minnesota men" were indicted in April 2015; eventually 10 in total were charged with seeking to leave the United States to join ISIS in Syria.
There are several problems with the Syrian refugee vetting process. Some administration officials have complained. But there are still glitches. The LA Times reports that federal officials have to reopen many of the cases. This reporting occurred 1/25/2017 so it is very fresh.
Federal agents are reinvestigating the backgrounds of dozens of Syrian refugees already in the United States after discovering a lapse in vetting that allowed some who had potentially negative information in their files to enter the country, two U.S. law enforcement officials said.
Agents have not concluded that any of the refugees should have been rejected for entry, but the apparent glitch — which was discovered in late 2015 and corrected last year — prevented U.S. officials who conducted background checks on the refugees from learning about possible “derogatory” information about them, the two officials said. At a minimum, the intelligence would have triggered further investigation that could have led some asylum applications to be rejected.
Fox News reports about actual terrorists coming from Iraq last year.
Two Iraqi men who allegedly lied their way past U.S. immigration officials and continued their terrorist-related activities after being admitted as refugees are the latest evidence that a flawed screening process is putting Americans at risk, critics say.
Aws Mohammed Younis Al-Jayab, 23, moved to the U.S. in 2012, only to return to the Middle East twice to fight for Al Nursra, was allegedly recorded by the FBI boasting about executing members of the Syrian Army and their Russian allies. Wiretaps, made while he moved from Arizona to Wisconsin and then California, captured him stating he wanted to return to Syria because he was "eager to see blood.”
CBS News found other examples of the failed Iraqi vetting process.
And ABC found more Iraqis terrorists in Kentucky.
Libya is now a failed state, "devoid of a functioning government" according to the Telegraph. Little chance that they can partner effectively in determining terrorist ties before entering the U.S.
Sudan (al Qaeda haven) and Iran and Syria
These countries are states that sponsor terrorism as defined by the US State department. Being so designated points to difficulties in partnering with them on proper vetting. Also they can be home to al Qaeda and other terrorist groups, like Sudan is.
Yemen--Proxy war Saudi Arabia and Iran
Yemen's own refugee camps have become compromised. Telegraph states
Yemeni officials have claimed that members of the al-Shabaab terrorist group have been arrested in refugee camps for Somalis. The government fears that refugee camps such as Al-Kharaz, which now houses 18,000 out of an estimated 2-300,000 Somali refugees in Yemen, could become recruiting grounds for radicals.
Officials also claim there are “regular links”, including arms transfers between al-Shabaab and al-Qaeda in the Arabian Peninsula, the group accused of planting parcel bombs on planes last month.
Pakistan and Afghanistan are more problematic
The deadliest single attack was at the Pulse nightclub in Orlando last June, which left 49 dead. The man responsible was Omar Mateen, whose parents were from Afghanistan, though he was born New York. Afghanistan is not on the White House list.
In a few cases, foreign-born nationals have carried out lethal attacks.
They include Tashfeen Malik, who, along with her husband, was responsible for the San Bernardino, Ca., shooting that claimed 14 lives in December 2015. She was born in Pakistan, but spent most of her life in Saudi Arabia until she came to the U.S. in 2014 on a fiancée visa to marry Syed Rizwan Farook. He was born in Chicago, to a family originally from Pakistan. Neither Pakistan nor Saudi Arabia are on the White House list.