My guess is they think the combo might be more effective than just one med, alone. My other guess is they are just looking for an excuse to deny you.
Their decision needs to be based on mainstream medical science - clinical trials showing evidence that combos are definitely better than meth, alone. I haven't been able to find any research that shows clear evidence, from a clinical trial, that a combo of meth-humira or meth-remicade is better than meth alone.
As the article showed, at one time, for instance in 2006, there were top docs who thought methotrexate mixed with remicade might be more effective than just one of them, alone. But the research presented in 2008 let the medical profession know this combo is not more effective, at least not for Crohn's.
Have you called the insurance company and asked it how to appeal the decision?