Because it would improve agricultural production and probably lead to more people. Which is in their view the ultimate evil.
The apex of the Medieval Maximum tracks quite well with the apex of the Cahokia Culture in the Midwest, and the Zimbabwean culture in Southern Africa.
I've been to both sites over several decades and noted the timeline of both.
Later, upon reading of the Medieval Maximum, I remembered that an explosion of people and trade had occured in both areas at that time.
Obviously, this time also corresponds with the settling of Greenland by the Norse.
You may think of other cultures that flourished during that timeframe.