Stark was written out of the series near the end of the third season so Paul Goddard could pursue theatrical acting. It wasn't the writers choice, but the actors. And he was still brought back to the series several times in season four, when his schedule permitted.
The season one DVDs are out of stock on Netflix, IIRC, due to rights issues. But that should be resolved very soon when the complete series is re-released on DVD (or has that already happened?).
And didn't Stark at least get a throw-away line at the end of season one explaining his absence? Or am I remembering a different throw-away line explaining another one of his absences?