According to "The Scotsman":
Mackenzie had his reasons for leaving. "I actually thought there was nowhere else for my character to go," he explains. "My character reluctantly arrived from London at the beginning of series one to take over the bankrupt estate and decide which girl he was going to marry. By the time I left, the estate was successful, I married the girl and reconciled my relationship with my father. It seemed appropriate to leave when I did."