AOE training at...

There are teleports between the platfoms. Time Temple was the first area designed with 4th job skills in mind. You can't spam ultimates in one place and hit everything, but you can move back and forth fairly quickly. Higher HP and EXP also means less moving around required. It's not all about exp ratio. If I 2hko Skeles for 4500 exp and 3hko Captains of Oblivion for 7k, I'm going to want to kill Captains.

Showa came out with a nerfed Armory, and we had higher level spots already. Time Temple isn't the same deal.

Not to mention an AM could train at Regret Mobs with just CL and ifrit if you wanted to. 1.5X damage. In fact, if the exp/hr isn't absolutely horrible, I might train there because it would cost signifigantly less than training at Newties.