There were many constraints, terrain (flat - IOW a Mare) certainly being one. Then orbital issues due to the need to support a free (or nearly free) return to earth (which implied a lunar orbit pretty much in-plane to the moon’s orbit). Those limited the landing latitudes because of LEM range limitations and the need for observations from the CM
Next there were issue with site lighting (they wanted the landing to occur with the Sun 5-13 degrees above the horizon to best illuminate/contrast obstacles on the surface), the site had to be on the earth side and in sunlight for the entire stay, and then there were technical issues with related to the navigation system that required a certain amount of time for alignment during the descent (during which the LEM had to be in communications with mission control, hence around the "edge") which basically ruled out anything further east than 40 degrees.
And I'm sure I'm forgetting some parameters.
Basically this left a section about 90 miles either side of the equator and about 1500 miles long. And the flat spots in that limited it further.
The parameters were planned to be relaxed a bit for later missions, but those never flew.
All of the missions had several possible landing sites, spaced in such a way that you could get several launch windows in during the right part of the lunar day cycle (spaced far enough apart to allow for the two day Apollo/Saturn recycle – IOW, about 12 degrees).