What's changed are three things:

• There used to be an upcharge for long-distance telephone calls. So even though telemarketing calls existed, they wouldn't be long-distance calls from some call centre across the country because that would be prohibitively expensive for the marketer.
• Calls used to be metered and charged by the minute or by the call. Every time a call was connected, the clock started ticking and the phone companies started billing. That means it wasn't economical to make thousands of bulk cold calls because you'd have to pay a nickel per minute and that would cost a lot of money and labour. On top of that, the people you'd call would get angry at you for wasting their airtime (especially on cell phones) and thus would likely not buy whatever you're selling anyway. On top of that, angry people would sometimes get revenge by faxing you pieces of black cardstock.
• The telephone network was analogue and physical. Nowadays you can outsource cold calls to a foreign country and sign up for a VoIP service that lets you make hundreds of calls a day through automated dialling completely anonymously, whereas just a few decades ago, you'd have to purchase a physical dialling machine for hundreds of dollars, hook it up to a physical telephone line, and call customers knowing that they can trace your calls back to you, and on top of that, successfully sue you for $500 per unsolicited call (in America) under the Telephone Consumer Protection Act 1991.