Simple. Too high in sugar.

Dr. Jeff often mentions that oranges have fungal/mold growth and would most-likely have a greater chance of causing an allergy while on the plan.

Oranges can contain a high fungal load, but its all the exposure to fungicides and pesticides from oranges and orange juice that creates such a strong mucous reaction which can interfere with the elimination of fungal candida. Traditionally, oranges have been heavily sprayed in order to bring them to market. Only in the past 15 years, or so, have organic oranges been around. Most people have a long history of commercial orange juice intake, which creates an allergic response that is difficult to correct and may take several years of not eating oranges to balance.