I concur with Tet68, either the US team and/or the Niger troops got complacent about the routine trips and didn’t plan for the ‘just in case something happens’.... “Intelligence Failure” would not be at the ‘national level’ because this was most probably a locally planned ambush and thus nothing for “intelligence” to find out about through various “intel assets.”
Czeck Resistance fighters successfully ambushed Reynard Heydrich, one of the most evil Nazi leaders, because he had a fixed routine that they figured out. Looks like the same kind of thing happened here. To stay alive, it’s essential to mix and match.
Don't ever march home the same way. Take a different route so you won't be ambushed.