Why don't Krishna devotees eat eggs?
Devotees of Krishna refrain from eating eggs because they are unfertilized embryos. They are largely artificially produced, with great neglect and torture to chickens. Chickens naturally create eggs, which in turn are meant to naturally become chickens, but that process is aborted in order to facilitate the sale and eating of eggs. Since persons who are sattvic ("mode of goodness") approve of neither abortion nor the eating of aborted and/or unfertilized offspring, we don't eat eggs.
Ultimately, the reason Krishna's devotees eat or don't eat something is because Krishna gives a list of what we should offer Him. Eggs are not on the list. Therefore, we refrain from eating them because we offer everything we eat to Him first, and take only the prasadam (spiritualized food – "the Lord's mercy") left after the offering.
Hope this is helpful.