lumpthing
generic lump
Are there better leaders for the Aztecs? Yes, most certainly.
Does the average casual gamer know them? No.
There's your reason. Personally, I would prefer Itzcoatl, Ahuitzol, or even Tlacaelel. But most people would not.
Firstly, only offering leaders that the average casual gamer already knows about is just lame. People, especially civ-playing people, enjoy discovery.
Secondly, Firaxis doesn't think this is an important principle. If they did, the leaders of China, Japan and Arabia wouldn't be who they are now.
So the choice of Montezuma is just a mystery. As Inhalaattori points out, it's possible that they mean Montezuma I, but this isn't at all clear. I suspect the Civilopedia will reveal that it is still the same old Monty II.