Pernell Roberts looks more like and older BJ than Trapper

Do you think the casting directors made a mistake, that they were so ignorant of the original MASH, that they thought BJ was actually Trapper?

It frustrated me when that show was on that Pernell looks like BJ rather than Trapper. I don't understand the casting decision. Perhaps it should have been called "B.J. Hunnicutt M.D."