I'm starting to suspect this as well, that we might not be seeing any C-Store sales anymore. After all, Cryptic already has your money the minute you buy any Zen. The only thing a sale does is give you an incentive to spend whatever Monopoly money you're still sitting on and technically that doesn't add to their profit margins at all. So while I can understand the reasoning, I still find it to be a disturbing trend if it's true.
What about those who don't have any zen and would buy some only if there was a ship sale? Totally silly to think there won't be any more ship sales IMO.