Why did MacDonald think Holmes was making a joke by telling him about the manor’s historical background?

MacDonald thought it was a joke because he didn't believe that anything from the past could have bearing on the present. He felt Holmes was wasting their time.

“It immensely adds to the zest of an investigation, my dear Mr. Mac, when one is in conscious sympathy with the historical atmosphere of one's surroundings. Don't look so impatient; for I assure you that even so bald an account as this raises some sort of picture of the past in one's mind. Permit me to give you a sample. 'Erected in the fifth year of the reign of James I, and standing upon the site of a much older building, the Manor House of Birlstone presents one of the finest surviving examples of the moated Jacobean residence—'”

“You are making fools of us, Mr. Holmes!”

“Tut, tut, Mr. Mac!—the first sign of temper I have detected in you. Well, I won't read it verbatim, since you feel so strongly upon the subject. But when I tell you that there is some account of the taking of the place by a parliamentary colonel in 1644, of the concealment of Charles for several days in the course of the Civil War, and finally of a visit there by the second George, you will admit that there are various associations of interest connected with this ancient house.”

“I don't doubt it, Mr. Holmes; but that is no business of ours.”

“Is it not? Is it not? Breadth of view, my dear Mr. Mac, is one of the essentials of our profession. The interplay of ideas and the oblique uses of knowledge are often of extraordinary interest. You will excuse these remarks from one who, though a mere connoisseur of crime, is still rather older and perhaps more experienced than yourself.”

he thought it is of no use and he thought that holmes was just trying to change the topic