Why isn't Saudi Arabia on the list?
So? Obama isn't president anymore. Trump campaigned on Obama's policies, especially his foreign policy, being ineffective. If anything this shows laziness on behalf of the Trump administration – not a good sign.
This, of course, excuses the action not at all. But it does explain the choice of countries.
(and, of course, Obama didn't tell Trump which list to pick.)
Saudi Arabia is a close ally of the US. That's the really horrific truth, and that's why Trump has business there and not in countries which the US is less friendly with.
This is why Trump's refusal to divest himself from his businesses is an unacceptable conflict of interest even if he insists it is fine and there are no grounds to question it.
"Have you traveled to, or been present in, Iraq, Syria, Iran, Sudan, Libya, Somalia, or Yemen..."
[1] https://www.cbp.gov/travel/international-visitors/visa-waive...