It's because citrus at high concentrations kills earthworms. Citrus in compost in normal quantities relative to other compostables seems to be fine, but you shouldn't be trying to compost a huge pile of just pulp and orange peels in your back yard.
As for why this worked here, I'm sure there are a whole lot of things that aren't earthworms living in a formerly rainforested spot in Costa Rica which can break that stuff down over 15 years.
https://explosm.net/rcg/vzyafxanj