Schneier isn't involved in cryptography enough today to be able to write the kind of crypto book people would expect. For example he knows very little about ECC, and stream ciphers aren't really on his radar either. Also he said on his blog that he'll probably never write another crypto book.

What do you wish to know? Whatever it is, there are already high quality sources of information out there right now. If you say what you're interested in people can point out good guides.

I completely agree! How could we encourage him to update the book?

While not a direct successor, there's this: https://www.schneier.com/books/cryptography_engineering/