Why did the delegates at the constitutional convention insist on meeting in private

They were afraid of losing their states rights

Quote by George Mason

"It is expected our doors will be shut, and communications upon the business of the Convention be forbidden during its sitting. This, I think, myself, a proper precaution to prevent mistakes and misrepresentation until the business shall have been completed, when the whole may have a very different complexion from that in which the several crude and indigested parts might, in their first shape, appear if submitted to the public eye."