The executive producer was Arne Sultan,instead of the late Leonard Stern,as Stern had been the executive producer in earlier seasons.Why was this?

This is just a guess, but maybe Leonard Stern was busy with other things so he handed off executive producer duties to Arne Sultan. It seems like a logical explanation anyway.

On the DVDs there is a interview with him. He said something like he left after a year or two because he given up on the show.