I can still see Barnacle Bay as being available at the Store...? Store->Worlds->Complete Barnacle Bay Adventure... you have to purchase Sim Points to get it, but it's still there. Maybe it makes a difference that I'm an Australian user?
The problem isn't that it's not listed in the store, it's that you can't buy it with SimPoints. When I view it in the store, it only has a button that says "Redeem Code." When you do a search for it, there is no SimPoint price given. There is no option to buy it with SimPoints on the page, either. I even changed my region to Australia, and it still wasn't available. If you see it as purchasable with SimPoints, how many does it say?
And yes, you can buy it directly from EA's Origin website or from Steam, but you can't use SimPoints there either.
When I looked on the Origins website, Barnacle bay was being sold at a reduced price (about $16-$17). The only reason I could see that they don't offer it for purchase with SimPoints any more is to make putting it on sale easier. It would also simplify buying it, since it eliminates the step of purchasing the SimPoints.