Because they do something every week. This week, until the 14th, there's a special truck store set up in DTD with vintage tees to sell, apparently. They already did long lost friends week at WDW, but there's no announcement yet about what's coming next week. I think it's usually announced on Friday. A lot of them have been disappointing, although I will say we really enjoyed the Easter Egg Hunt at World Showcase a couple weeks back!
I haven't heard anything on next weeks, but this weeks is the disney truck shop in downtown disney is selling limited vintage shirts. And in Disneyland they are having long lost friends week because it was very successful in Orlando.