I know double merles have health concerns but why?

I can give you an example that may answer your question. There is a breed of cat called the Scottish Fold, and their ears fold over. This is a cartilage defect. If you breed two Scottish Folds together, the cartilage will become too weak and cause other problems. Likewise, double merles have a genetic defect that was doubled by breeding two merles together. It has made their genes too weak and causes serious problems. It is like two people who have already had lung cancer, and lung cancer runs in their family, having children. The child will be twice more likely to have lung cancer as an adult.