The assignment of civ traits appear to've been based on three considerations:
i) "Realism". The historical Aztecs and Mongols were very warlike, and appropriately enough, they're Militatistic civs in Civ3.
ii) Match-ups with Wonder characteristica. Eg, many people have complained that China oughtn't be Militaristic, but having them so means they can get a GA from building the Great Wall, which indeed is a great Chinese achievement.
iii) Trait balance; some assignments are, apparently, mostly due to that Firaxis wanted all trait combos to be representated, and not too have too many civs with the same traits. The importance of this consideration appears to've been downgraded in PTW.
That America is Industrious would seem to fall primarily under i). It's the greatest industrial power yet, and appropriately enough an Industrious civ in Civ3. ii) may also be involved - the Hoover Dam is an Industrious Wonder.