Good question.
Maybe people fixated on eggs for a combination of reasons:
- Eggs are in common (daily) use with a (relatively) constant demand (its not like something you buy once and then forget for the next half year)
- 'Brands/types' are less important. an egg is an egg, not like, for example, breakfast cereal where you have dozens of 'types' (bran flakes vs. cherrios) and manufacturers (brand name vs. no name) Yeah, you do have specialties like 'organic' or 'brown' eggs, but its not as important
- Unlike something like gas, you wouldn't expect 'fluctuations' (with occasional decreases depending on the weather, what OPEC does, etc.)
- The regular price is an understandable number (not too high nor too low)
But most likely, someone somewhere fixated on 'eggs' in some speech and the issue just stuck.