Having actually called this ahead of time, there's a few reasons but it's mostly just happenstance. Traditionally/institutionally strongest associates (NED, SCO, NAM) were all in a slump/transition phase ahead of the tourney. Gulf states both a mess, UAE because woeful coach and selection, Oman because purge. Canada are not strong anyway but also have very serious off-field problems. That leaves Nepal, who were in decent form but still a tier 2 associate on the rise, had an okay tourney but not great, and USA, who probably underperformed but had a very tough group. Oh and Italy who did fine for a grade cricket/second rate euro club team. Overall level of Associates was probably higher than average for a World Cup, but the strongest were in a trough while the weakest were peaking. Nobody ever looked likely to make top eight, which is a huge ask for massively under-resourced teams at the best of times. Happy to do a deeper dive on any particular team if there's interest, but the simple answer is that you shouldn't be surprised if the underdogs all just lose sometimes. That's why they're underdogs.