No mandate would've been needed, because it was standard back then for teasers to be relatively short. The modern practice of putting 6-12 minutes worth of stuff before the title card/sequence would seem bizarre to producers from the 20th century, or at least pre-'90s. (Monty Python's Flying Circus even did that as a subversive joke a few times, holding off the opening titles until the middle or the end.)
The thing is, there's always been competition to capture the audience's attention right away and keep them from changing the channel to something else. That's why cold opens were created in the first place, and why many older shows would air previews of the upcoming episode right at the start -- they were hooks to get the audience interested in sticking around. But as cable channels proliferated and the competition grew, the pressure to hold the interest of viewers with shrinking attention spans led to main title sequences getting shorter or abandoned altogether and cold opens getting longer.
Although along with the longer cold opens, we've seen the proliferation of serialized shows that open with mythology intros (e.g. "My name is Oliver Queen/Barry Allen/etc.") and "Previously" recaps, so we're back to having shows that open without new content right off the bat. I guess with DVRs and streaming and such, the competition to keep people from changing the channel has subsided. Also, shows are more serialized, so it's more important to recap the past than to lure the casual viewer into watching the current installment. But the custom of long cold opens has become routine enough to continue on its own. Maybe it's just that the recaps and such take up the real estate that used to go to a brief teaser, so the modern cold open needs to be approached as just the first act of the show.
