Why?! Don't know. Probably its a marketing thing (or lack thereof). When I buy a Judge Dredd book I can't put it down until the very last page (usually this is true), but after that I never hear any new news form 2000AD publicizing the next book. This turned into a (first world) problem when I stopped shopping at traditional brick and mortar stores.
The point here is, going into this book, I had no idea what Day of Chaos was. But this books does a good job summering it up for me. An enemy Mega-City (East-Sov) made a (successful) biological attack on Mega-City One. The majority of the population died and Mega-City One is in ruins. And everyone blames Dredd. This is the story of what happens next.