This surprised me, as I'd heard otherwise. Per https://www.sarahwoodbury.com/how-tall-are-you/, "the average height of people who lived in the 9-11th centuries was comparable to ours today. It then declined slightly during the 12th through 16th centuries, and hit an all-time low during the 17th and 18th centuries – when those doorframes were made."

Perhaps they wanted to keep the heat in.