Because those were implemented in later Patches.
Vanilla initially only had MC and Ony as raid, any other raid was introduced in a later patch.
SSC, TK and Hyjal were open since Day one of TBC, your only limitation were the Attunements to enter them.
Sure, but there also have been quests (or entire hubs) that were later implemented in Vanilla and still have been in Classic right off the bat.