It's easier from a technical standpoint to focus on a single-platter drive and get it working, with high yields, anytime you have a new areal density jump. From a consumer standpoint, it's always nice to have a range of capacities to choose from, but it's not always possible to time it right. If the single-platter is ready for shipping but the multi-platters aren't, do you delay the whole family or only ship the one that's ready?
My gut says the lack of fanfare is related to having uncertain supply/quality. Make a new product, send it out to distribution and see how it fares. If the return or failure rates are high, it's easier to quietly make changes than if you have a huge advertising campaign and then have egg on your face.
Or maybe the Marketing Dept had its budget severely scaled back.
IMHO your first suggestion is the most likely one. Marketing wise, it doesn't make much sense to hype up a 500GB/platter drive if the only one you have is 500GB. Better leave it until you have the larger ones.