As said above, CE was constrained by the technology of the time. The Elites just weren't as detailed as they were in the later games. That goes for all the enemy units in fact. I distinctly remember seeing a CE Wraith again after playing H3 and thinking, "wow that's just a lump of purple".
Not to mention, in H2 we got cut scenes that were centered around the Covenant, which served to provide us with better details and just straight up more exposure to the Elites and their appearance. H3 continued this trend as well with the Elites being allies with humanity. H4 then had far better graphics, and thus could show us more detail. H5 has the same graphical benefits but also features Elites heavily in cut scenes. So really, just the exposure to Elite designs in cut scenes probably contributes to your better recollection of their designs. When watching a cut scene, you aren't focused on your sights and trying not to die. You're just watching. So your eye wanders around the screen and notices more nuances in the designs of character models.