I think because it shares a long geological history with the European plate, and the history of movement has been more or less rotational in-situ, and not as an independent plate with a distinct geological history. Take as an analogy the Anatolian plate, which is often considered a part of the Eurasian plate, "In some references, the Anatolian Plate is referred to as a "block" of continental crust still coupled to the Eurasian Plate. But studies of the North Anatolian Fault indicate that Anatolia is de-coupled from the Eurasian Plate." Which would suggest the physical difference would rely on being decoupled from the parent plate. However it strikes me also that this can be a linguistic matter for ease of communication, it may just be a matter of convenience or description of Earth history, much in the same way you might talk about the Avalonia plate, or Baltica plate.
So I think Iberia is a sub-plate or block, because of the shared Geological history with the Eurasian plate, rather then the independent history (Gondwana and India separately) seen within the geological history of the Indian plate.