They were probably planning to make quite a few with the first or second wave of 2014 sets to coincide with the Ep.III re-release, but since that got pushed forward to this year, and then postponed indefinitely (if not altogether canceled) Lego may be changing the sets that get released. If you want to blame anyone, I suppose you could blame Disney. Hopefully it was such a late change that Lego releases any Ep.III sets they had in the design phase anyway.
But really, by my count there have only ever been 9 sets released with proper Phase II helmets, and 3 of those 9 were released this year, so this has been somewhat of a banner year for them already. They'll make more eventually, but they're most likely to release a certain type of set in large numbers if it coincides with the release of something or some ongoing project.