I'll try to kep this simple, but don't worry if you don't fully follow the reasoning.
Religious Jews cannot drink grape based bevarages (e.g. grape juice, wine, brandy, sherry, port etc.) unless it is certified kosher. For this to happen it basically has to be made by Jews. If you feel this is racist, you're right, you should taste how foul Kosher wine can be
! But seriously, there is good theological reasoning behind this, consult your local Rabbi for a full explanation.
Following on from this - there is obviously a question of whisky being, for lack of a better word, "contaminated" with non kosher sherry, or port, or whatever the next crazy finish is. Different local Jewish authorities (Beth Din) hold by different views - as I stated before, the London one believes that the effect of the sherry is not sufficient enough to cause a problem. Most others, especially in the US, feel that the effect is too great and makes the whisky non-Kosher. Having had experience of cask effect though, I would reluctantly agree with the latter opinion - I don't believe differences can be put down to the different oak alone.
Having said all this, if a distillery could get its hands on casks that held kosher sherry in, they would have a very large and grateful market to sell their whisky to. Unfortunately though, the only kosher sherry that I know of is a light fino, which may not have such a good effect on a whisky.
Hope this makes things slightly clearer,