From what I understand, their network was overengineered and they generally put out a very high quality product, and didn't need to do a lot of marketing.
They put out a high quality, overengineered, extremely expensive product that, by government mandate, had no competition. After deregulation it was clear that customers wanted a little less quality for a lot less money.
You don't need to do any marketing when you're a government-protected monopoly and customers have a choice between having phone service with you, or no phone service at all.