Why a "plan B" and not a "B plan"?
While there is no "rule", one can obtain some guidance from observing that, in "plan B", we are not speaking of a plan named "B", but rather simply the second plan in a succession, using the alphabet to annotate them. Had we wanted to name the plan we might have used "B Plan" (though the specific words "plan B" are so idiomatic that this one isn't going to sound right).
Consider "The A Team" (the name of a team) vs "team A" (simply the first team in a enumerated series).