Why would some members of the Continental Congress be opposed to signing the Declaration?

One of the most famous reasons for why certain delegates didn't sign was that the document lacked a legitimate Bill of Rights which would protect the rights of States and the freedom of individuals. Three main advocates of this movement were George Mason, Elbridge Gerry, and Edmund Randolph. Hope this helps!

since most of current immigration into the u.s., both legal and illegal, is coming from latin america. it would be very difficult to imagine or rationalize a country my fellow citizens are trying desperately to flee into, would pose a threat to my prosperity or freedom. since the u.s. has a long history of intervention, but no record of permanent occupation, i would not feel oppression is the main objective.  

the largest free trade nation, along with the largest gdp in the world wanting to expand it's economic policy on us, and including us, could only be positive. the u.s. government has no interest in, or way of simply forcing american culture on us. and has no track record of it. other nations that adopted american social culture, or "americanization" did so voluntarily, mainly through the choice of allowing american businesses and u.s. citizens to come into their nations, as free operatives.

missouri compromise of 1820 which prohibited slavery