I'd also love to work on such a thing, but even so I wouldn't make it on spec (e.g. to sell as a generic tileset).
I was going to say that perhaps it's because of the difficulty of making such a tileset sufficiently generic to sell, since fantasy is a genre largely defined by its settings and details. However, there is no shortage of generic fantasy top-down tilesets (perhaps thanks to RPGMaker setting a standard for required assets with its basic package).
So, I guess it's like MM said - there's not much need for such tilesets these days, so it's not worth the risk to spend the time creating one. Most current isometric games are either 3D or have a simple enough visual style that they can quickly create the art in-house. The isometric fantasy RPG genre is also largely defined by 3D games like FFT, which may be why people aren't making a lot of 2D ones.