The Stoic Resilience of PDF Within a Digital Ecosystem

It's like asking why HTML hasn't been replaced. The business value needs to be greater than the switching costs for both publishers and consumers.

However, as someone who spent the last 5 years working for PDF companies, I predict that a new final form, fixed layout standard will take hold within the next 5 years. The format will be able to adjust itself to multiple form factors. Both Adobe and W3C are working on competing standards currently.