Why would they announce the villain now? The fan speculation has been flooding the internet for months (years?) and the vagueness of the trailer has only heightened it. The tidbits of information fed to us by JJ, Orci, and Lindelof are meant to keep us guessing and thus keep us talking about the new movie. JJ et al. know exactly what they're doing from a marketing perspective.
Exactly. Even mainstream media like Entertainment Weekly
have been running articles speculating about the villains and the mysteries of the plot . . . .