Long-distance calls in those days were placed through operators and were expensive: $1 a minute when average earnings were maybe $50 a week. The first direct distance dialing call was not placed until 1951. They weren't available in major cities until the late 1950s and countrywide in the 1960s.
In those days, politicians outside of Congress didn't see much of each other in person. Train travel was time-consuming and plane travel hazardous. Regularly scheduled jet travel only began when the Boeing 707 was launched in 1958.
It's no coincidence then that the last multi-ballot national convention was in 1952, when Democrats nominated Adlai Stevenson. As long-distance calls and jet flights became more common, some of the communication that could occur only at the convention started happening earlier.And since party bosses no longer choose delegates, they can no longer deliver them, either. Accordingly, if there are negotiations, they will take place among many different people via electronic communication long before the convention.