Why was the delegates' meeting for the Constitutional Convention so secretive?

At the Constitutional Convention of 1787, the Founding Fathers' deliberations were held in total secrecy. The reason for this is that those attending the Convention did not want any information getting out which might lead to misunderstanding or the deliberate distortion of what they were doing. This may seem like an overreaction, but these concerns were perfectly valid under the circumstances. The political atmosphere was deeply acrimonious at the time, with people holding many different competing visions for what the United States should look like in future. The fierce partisan debate was reflected in the press, which routinely engaged in outright slander against individuals whose positions they disagreed with. In such a toxic environment, it is not surprising that the delegates of the Convention agreed to hold it in secret.
Secrecy allowed the delegates to have a vigorous, open, honest debate without worrying about what their political opponents and scurrilous journalists would say. It also meant that some of the heat could be taken out of an increasingly fractious political environment, greatly conducive to the overall stability of the country. The making of the Constitution was an important business and needed to take place in an appropriately serious atmosphere, one in which differences could reasonably be aired without being subject to the dictates of ill-informed opinion.
http://law2.umkc.edu/faculty/projects/ftrials/conlaw/convention1787.html