I speculate in print (in an upcoming article) that it was a political decision. Figure it this way. Enzo had already had his hand slapped by the FIA in previous decisions (such as not homologating the 250LM when it first came out). Who is in a position to know what the FIA is announcing next? The press. The first customer for the car, Mr. Conti, was the publisher of an important car racing magazine. He would be the first to know if the FIA was about to announce a decision that would affect Ferrari's racing future. Even to get one day's notice in advance would allow Enzo to come up with a counter strategy, so having the body panels of just one Daytona coupe made in alloy so the publisher would have something special to brag about was a small price to pay if he gets the right tip at the right time.