WHTW I think has two main issues really; firstly a smaller team and thus a less complete mod (ancillaries, general pictures, unit cards, that sort of thing just don't get done, so the mod looks more incomplete), and secondly that it only appeals to those who like Warhammer and TW, whereas Amazons appeal to all the TW fans. It's a massive achievement for any mod for a game eight years old to make the top ten, and Amazons TW is one of the most professional looking and diverse mods for RTW out there so very worthy of it.
As for WHTW I'm hoping we'll hit top 100 when we do our next (probably final) release, but we'll see.