It's pretty consistent that people think of (and therefore design) brutes and soldiers as having high fort and skirmishers as having high reflex. Artillery can be all over the place (I'd say high reflex is most common but it's nothing like the overwhelming ratio of the other three big archetypes as artillery is a mix of archers (reflex), rock hurlers (fort) and spell slingers (will)). The two small archetypes - controllers and lurkers - tend to be high will (especially for controllers - lurkers are often reflex-y) but are much rarer.
And IIRC the WIR Monster Vault on RPG.net counts the number of monsters with fort as the highest NAD. And it fits there too.
Shorter me: Big dumb muscle has high fort. Most monsters-of-the-line are big dumb muscle. Leaders and brains behind the organisation are rare and those are the monsters with high will.
(On a tangent, really low level monsters tend to be small (goblins, kobolds) and therefore high reflex, low fort (and also low will - thought to be stupid)).
Edit: Even shorter me: If the monster is a threat because he's bigger and stronger than the monster designer he has high fort. If it's because he's faster and more agile or skilled he has high reflex. And smarter implies high will. Most people don't write many monsters as smarter than them. And most people remember being kids when everyone was bigger than them, so that's easy to write.
Edit 2: Apparently "Even shorter me" isn't.