Why doesn't Mac sleep when closing the lid after screen replacement?

This could certainly be a screen issue. Lid open or closed detection on this model is done by magnetic sensors on the logic board which are positioned to read magnets in the display assembly. The magnets could be in the wrong location, or the screen you used isn't 100% compatible with your specific model. Sleep/wake issues are common in those cases.

If you have access to a magnet, see if you can locate where on the screen the magnets are mounted (or if they exist). by running the magnet along the left and right edges of the display. There should be one on each side. You can also see if you can get the display to sleep by using the magnet to trigger the sensors on the logic board.