Appleby does not specifically address this issue, but it is clear that she does not see the American Colonization Society as a viable vehicle for ending slavery. The society had a great deal of support from the public and was only able to relocate 3,600 African Americans to Africa in over 40 years of operation. This does not seem to indicate that the society would have been able to have any significant impact without radical changes in the surrounding situation (like a major influx of government money or a change of heart on the part of slaveholders.
The society could not really have played a bigger role because neither African Americans nor slaveowners were very interested in colonization. Free African Americans typically did not want to go live in a place that was strange to them. Slaveowners were interested in colonizing free blacks, but not in freeing and colonizing their own slaves. So long as these conditions held, there was no way that colonization could work as a solution to the problem of slavery in the US.