It took like 5 months to cool web to operational temperatures rolling the telescope would create so much heat all new images would be useless until it cools down again.
Though I suppose now that I think of it, it's possible the main mirror assembly actually has no built in roll control but only pitch, since the yaw part could be done by moving the entire telescope while remaining shaded. I've never seen any videos showing the full movement, but the previews for LUVIOR show it having full 3 degree articulation relative to the heatsink segment, so I assumed the Webb also has it given that they're extremely similar designs.
It's in an orbit around L2, so it's not statically positioned in space. L2 also moves with the earth around the Sun, so it's not statically limited to any one region of the sky.