Most people's early mobile plans were very limited, the person receiving the call paid for it in the US, and costs were even higher for calls that weren't from your immediate local area. And, as you say, junk landline calls were becoming a bigger deal.

This old article https://slate.com/news-and-politics/2001/06/why-isn-t-there-... claims that the wireless carriers didn't want to put out a directory because consumers would be upset if they had to pay for junk calls.