Something to note is that in the 1950s, television was still very new. No one was sure what it was, what it would become, or how to deliver content through it. As a result, they went with the closest analog they had: stage theater. If you watch old movies (Cat on a Hot Tin Roof is one of my favorites,) you'll notice that it "feels" much more like a stage play than movies of today. This is deliberate. Language evolution is most likely not the reason they sound different to us, as you proposed.
There is also the matter of old school sound equipment, which was fuzzy and limited. It required very explicit pronunciation in order for the audience to understand. This is why the people tended to enunciate more clearly and speak with an affectation that sounds strange to our ears. Most of them took classes in order to learn how to speak with a "television voice" designed to sound intelligible on the terrible speakers that existed in consumer-level appliances. Even celebrities appearing on talk shows and in candid interviews would often use this voice. It is not at all indicative of how everyday language sounded in the 50s.
I know this doesn't answer your question, but it is important to understand that people in the 1950s were generally not more eloquent than we are today. In fact, there were most likely more distinct dialects back then, many of which would be considered today to sound ineloquent, uneducated, or even unintelligible. Your perception is skewed by an often wealthy and well-trained minority speaking with an affectation specifically designed for television and radio. Specifically, your example shows a group of people who, even in their time, would have been considered to be exceptionally well-spoken and to possess a larger than average vocabulary.
Today, you'll notice that millennials actually tend to have less regional dialect variation than their parents or grandparents, thanks to the proliferation of media. This is especially noticeable in the Southern US. Celebrities on TV today sound less "eloquent" than celebrities in the 50s because we've done away with the theatrical training and the "TV voice". There are some exceptions, but the majority of celebrities today speak with the same voice whether on TV or at home.
I hope this helps a bit.