When developing FoV, saved game compatibility was of utmost importance, and anything that would break that was not included in any updates for the D2:ED section of the game.
DKS was primarily intended for people who did not already have D2:ED, so saved game compatibility was not as much of an issue. When there were changes Larian wanted to make to DKS, saved game compatibility was an easy sacrifice to address various issues and add various improvements (though IMNSHO the change to the human target lock system wasn't an improvement).
Lar did mention (once) the possibility of a tool to import a D2:ED save in the Hall of Echoes and export it to the FoV section of DKS. Presumably they didn't think there would be much demand for a built-in import function, or it wasn't a high enough priority to get implemented before release.
For what it's worth, there are plenty of bonus stat and skill points to be had in FoV from mindreading, so starting off with a new level 35 character isn't going to put you at a disadvantage (if you don't want to try a different build in the remastered D2:ED). You would be upgrading your equipment fairly soon in the add-on, anyway, so its lack doesn't really matter. There are additional skill trainers, etc, in Aleroth, so you may not even need to return to the battle tower, so possibly different servants isn't that big a deal (though you may particularly like or dislike specific servants). It would be nice if you could carry over your previous D2:ED saves into DKS, but you do get pretty much the same experience with a new character.