Well, printing can be quite expensive. Distributing new directories can be quite expensive. Either trashing or recycling old directories can be expensive. Compare that with a low end terminal that will last several years (instead of one year) and yeah, it may end up cheaper.

But this appeared to be much more than a directory. It offered many online services, services for which a fee could be collected. That is only marginally possible with a dead tree. (I seem to remember the front part of the directory containing dial-in information services in the early part of the decade. I never tried them, so I don't know what it was like, but it sounds analagous. You're just listening rather than reading.)