I'm starting to think Coachella has completely and totally fucking shit the bed on one or some of their bigger names. This is getting ridiculous.
I think that Coachella was banking on The Rolling Stones headlining. Once contract negotiations fell through, Coachella found themselves missing a headliner (and didn't want to go the Governors Ball route of advertising an "as-yet-announced third headliner"). When Black Sabbath pulled out near the lineup release date last year, Coachella found The Black Keys - a convenient and capable replacement - to step in. The Black Keys were already touring, so it was easy for them to add two shows. Such a replacement might not exist this year, hence the delay in the lineup release.
...or I'm completely wrong and Coachella is about to knock us out of our chairs with an amazing announcement.