This is a very good post. It involves many facets that are well-beyond what I'm willing to write out, but I'll address a few key points.
Genetics - as you mentioned, genetics play a huge role. Is the issue of genetics merely enough for some elite bodybuilders? Yes. Some are so damn genetically predisposed to gaining muscle that it's scary (eg, Casey Viator). I would consider genetics to be the most important factor. If something causes growth, why change it (providing the growth continues)?
Exercise Selection - some lifters, especially O-lifters, began their lifting career with exercises that cause profound muscle growth. Since these same exercises (O-lifts) tend to cause the slowest adaptation, said exercises can be performed for months without stagnation. In other words, a lifter will quickly adapt to preacher curls, whereas snatches vary enough from rep-to-rep to cause a slightly different stimulus. The issue of total muscles involved is also important. The more muscles involved in a lift, the longer it takes to adapt.
Therefore, some lifters can perform nothing but snatches, squats, presses, and pulls without ever stagnating. In fact, such exercises are recommended for that purpose.
- Training Goal - if a lifter is primarily interested in elite levels of maximal strength, exercises should be rotated more often. The reason? It's necessary for p-lifters to really push the intensity on certain key exercises. Once you push the intensity of a certain exercise for a few weeks, adaptations take place that causes quicker accumulation to the demand.
Once again, good question.