Why did the framers not abolish slavery at the time they wrote the constitution?

Because, at the time, the South strongly defended slavery and the framers knew if they abolished or aimed at slavery, the South may be upset with the Constitution and never agree to it. The North alone would never have been able to ratify the Constitution on their own, they needed the South, as well, so, in order to gain their full support, slavery had to be left out of the Constitution. They also assumed slavery would dissapate by the early 1800's.