Though I in no way condone slavery, there was a valid reason for those two southern colonies to say no. And, as in most cases, it's economic.
I'll limit my comments to Georgia, for obvious reasons
In the years prior to the Revolution, Georgia had barely been settled outside of the coastal zone. That coastal zone was (and is) an exceedingly swampy, malaria infested and heavily forested area. Even today, with second third and fourth generation planting, coastal South Carolina and Georgia evidences this in the areas that are not overgrown with beachside building. As any Marine will tell you who spent time at Parris Island.
Georgia was founded in 1730. Interestingly enough slavery was originally forbidden in Georgia, though it was chartered as a penal colony for what we would call "blue-collar criminals" today. It was almost impossible to get your average colonists to
voluntarily remain settled there- they chose to go further north, or if they were particularly hardy, further inland. Indentured servants and prisoners tended to run away. It was not a hospitable place.
As a result slavery was legalized in 1749. Slaves and prisoners could be forced into the back-breaking labor of clearing the land for planting. By 1775 (Only 45 years after the founding of the colony), it had become an economically viable area, through massive investment in slavery, with rice, sea island cotton and indigo production being the primary driving factors. These crops are not what you would call-labor friendly, especially in the pre-mechanized age.
Thus when the calls to abolish slavery started up in the Continental Congress, the very real questions of 1) who's going to reimburse the slave owners for their investment if they are emancipated? 2) how do you expect Georgia and South Carolina to survive economically without it? and 3) what's to become of slave population if they are freed? needed to be answered. And they weren't. There were no real, viable answers provided to these questions (though to be fair James Monroe did attempt to answer #3 in later years, giving us the modern-day failed state of Liberia). So without such answers to those questions Georgia said "Ah nope, ain't gonna sign that".
