Eberron was originally introduced during D&D’s 3rd Edition and remained popular even during the D&D 4e rule set. Releasing this update through the DM’s Guild instead via print will be sure to upset some fans, but make a lot of business sense from Wizards’ perspective.
1. It allows them to gauge interest in the setting without the massive costs of publishing a hardback book. Assuming interest is strong, Wizards may offer a hardcover print-on-demand version through DM’s Guild. (Mike Mearls has already confirmed a POD version will be available when the rules are out of beta).
2. It opens up another setting to the DM’s Guild creators aside from Forgotten Realms and Ravenloft. This is especially nice for those with magic-tech ideas that might not fit more traditional D&D fantasy genre.
This is a seismic shift for Wizards of the coast, since no other official 5th Edition product has been offered direct-to-consumer through the DM’s Guild. There are some drawbacks with this move, though.
As noted, fans of the setting may feel slighted that Eberron is not getting a print release while Ravnica is. Additionally, the Friendly Local Game Stores may feel a bit of trepidation as this is the first direct-to-consumer offering from Wizards of the Coast. Up until today, no official D&D 5th Edition product has been offered as a PDF. If I were a bricks & mortar retailer, I might be somewhat unsettled by this development (while feeling somewhat calmed that Ravnica is coming -- more on that later).
Other Updates to Come?
For my part, I am immensely excited at the prospect. I’m not a big fan of the Eberron setting, but this development means there may be hope for other 5th Edition updates that are unlikely to get a hardback book. Greyhawk, Dragonlance, Dark Sun, and Planescape are all likely candidates for a PDF-only release.
Greyhawk hasn't seen an official release for... well, longer than most people can remember. Dragonlance had a major update during the 3rd Edition and Dark Sun got its own 4th Edition hardback... but any one of these could be a nice outreach to fans of those settings. Given that psionics is openly being worked on at Wizards of the Coast, Dark Sun would seem to be a strong candidate for the next PDF update, and since it is not as "generic fantasy" as Greyhawk and Nentir, it doesn't compete as much with the Forgotten Realms.
Recently, Mike Mearls has been updating his “home game” version of the Nentir Vale document and showing the occasional sneak-peek on Twitter. These peeks look a lot like what one might find in an official update. A Nentir Vale update on the DM’s Guild would be an amazing win for fans of 4th Edition and the Vale.