It has become worse over the decades, but with the arrival of the telephone we have the opportunity to 'drop in' unannounced on our hosts. These new technologies provide us with the capability - not just the telephone, but instant messaging, e-mail, caller ID and many others.
Imposing upon our hosts without advance warning is considered rude (even today) because you put the host in an uncomfortable position: he may not want to receive you just then, he may have to adjust his plans to accommodate you, and so on. You are intruding into his private sphere and catch him unawares; you have the upper hand while he is disadvantaged. You, as the caller, know when the call will be made and what the subject is. The receiver is completely ignorant of these matters - all he knows is that the phone is ringing and it must be answered.
You would, hopefully, never turn up on someone's doorstep unannounced expecting your host to invite you in, devote his time to you, sit you down to dinner (unless you are on very intimate terms). Then why do something similar with communications technologies?
Today's polite society advises against taking advantage of the ability to 'drop in' without prior agreement. Technology gives you the possibility but that doesn't mean you should use it.