Well first off, not everyone in the colonies wanted to rebel, so to openly bring the entire population into the conflict may not have played in his favor. In fact, in the later part of the war, the British made this very mistake. After failing in their initial strategy to subdue the heart of the rebellion in Boston, then failing to succeed in New York, the British turned to what they thought was a hotbed of Loyalist sentiment in the southern colonies. Their presence and insistence on loyalty oaths essentially forced a large portion of the population that had until this point preferred neutrality to taking sides. This shift broke heavily in favor of the patriots, and the resultant swelling of patriot support in the region played a large part in Greene's ability to force the British into a protracted Pyrrhic campaign.

Even if Washington could have forced the entire population into the war, which is doubtful given his limited resources, there is little reason to think that the already fickle population wouldn't see this as a step too far and break heavily in favor of the devil they knew.