Why did Alastair Mackenzie (Archie) leave the show ?

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."

I have no idea. I am still wondering about that, myself. Maybe he was made another offer with better money or something that would better enhance and promote his career. Just a guess.

MacKenzie moved to Hollywood for about 9 months after leaving the series to seek other roles, but not having nailed any parts he moved back to the UK.