As long as people are speculating…
In the entertainment industry, everything hinges on agreements. There are union agreements and there are actors’ individual contracts. The placement, order, and presentation of credits are all spelled out in contracts. A big star may not agree to do a movie unless they get top billing. A director may demand that his name appear before a producer and the assistant director’s union contract may demand that the assistant director’s name appear wherever the director’s name appears.
The agreements of the time (which could have been a carry over from the days of radio) may have required the top stars’ names to be read at the beginning of the show.
I don’t know what the agreements of the time said, so I can’t give a factual answer.
By the way, ever wonder why ads (print ads and television commercials) for movies sometimes contain 500 names in small print and other times don’t even mention the name(s) of the star(s)? That’s because of various triggering clauses in contracts. If you mention the star’s name, the director’s contract requires you to mention the director’s name, then the assistant director’s union contract requires you to mention the assistant director, and so on down the food chain.