It's a bean-counting move (at least the Magic: The Gathering part, and probably Eberron, too).
Say they instead released an update of a really good setting, like Planescape, Greyhawk or Dark Sun. Many fans of those settings, if still active in D&D, would already be adopters of 5E due to its 1E/2E callbacks -- 5E is already much closer to the original than anything for the past 20 years. So by putting out a new <insert awesome 1E/2E campaign setting> sourcebook, they're basically getting some percentage of existing customers to buy one book.
With the Ravnica book, they don't really care at all about D&D fans, although some will gobble it up just because it's a new book with a little crunch. The target market is Magic players who can be lured into D&D -- each of whom represents 2+ books sold, as they'll need a PHB + Ravnica. And given the stereotypical mindset of CCG players, WotC's probably betting that most will pick up all the books, to get access to all the toys/crunch/exploits.
So that one, at least, isn't about pleasing existing customers but rather attracting new ones.
The Eberron decision is a little more puzzling, but I assume they have some demographic data suggesting late 3.5-era players who cut their teeth on Eberron are a prime market. Players who started playing as teens in the early-2000s would be in their early 30s now? Prime time for nostalgia + disposable income. Maybe there are more Eberron 3.5 holdouts, since it was never converted to 4E and doesn't naturally dovetail with 5E like the earlier campaign settings do?