Celebrim fundamentally hit the nail on the head. I will add a comment of my own....
To many Realms fans, Maztica was a atrocity and was resented because it basically a real world analog (Cortez and the Aztecs historical era) that changed some names and had the Forgotten Realms brand name slapped on it. Ed would say that some areas were 'certain similarities' to certain real world areas and cultures, to help the imagery of architecture and dress, etc.. TSR seems to have misinterpreted and would make an area 'exactly' like the area....example - Mulhorand = Eygpt, not Mulhorand has certain cultural and architectural similarities to Eygpt. With Maztica, TSR went too far with the real world analogy.
Most true blue Realms fans hated it and wouldn't touch it with a 10 ft pole.
And most of the non-Realms gamers wouldn't touch it with a 10 ft pole because it had the Forgotten Realms brand name on it. (Why is it that people either hate the Realms or love it....is it because the middle ground folk are quiet compared to the extremes? Or is it because the setting either developed a 'love it or hate it' perspective.
In short, it failed for the reasons that Celebrim indicated and the fact that a number of Realms people hated it and wouldn't buy it and the Non-Realms fans wouldn't buy it because...well, it had the FR label on it.