Wales was conquered by the Plantagenet king Edward I in 1247 while Cornwall had been absorbed by England by the 1050s. While 200 years may not seem like much in the grand scheme of things, it has within it one of the most important and well known dates in English history, the conquest of England by William of Normandy in 1066. Not only that, but there is little 'popular' history of England before 1066 apart from the Romans (who were around WAY before) and maybe Alfred the Great. As such, it is very hard to find people familiar with the idea of an England which does not include Cornwall in it.

In short, there is not much well known history of it as an area separate of England politically, let alone culturally.