Too hard to monetize, too high costs, competition by the phone company. Everyone had one, they were ubiquitous. Without the internet ads are much harder to se

Too hard to monetize, too high costs, competition by the phone company. Everyone had one, they were ubiquitous. Without the internet ads are much harder to sell, huge barrier to entry for customer acquisition compared to google. Without real time auctions and targeting ads are worth less, i.e. you can't target search terms in a phone book, only prefixes, let alone things like demographics. The cost to distribute a book to everyone is a lot higher than the cost to serve some traffic. I suspect ad density was too low too.