One theory is that interactions and collisions (all scales: gas, dust, comet, planet) are what cause the participants to align in the direction of the original net angular momentum, and the Oort cloud is just too sparse to that have happened as much.

Oort dust clumped up to comet-size objects for sure, but that tended to happen for the particles that were already roughly in the same orbit. Looking at all orbits in the Oort cloud, they remained more random.