I think only Gondor is mentioned in the movies is because it is easier to explain that he is the king of Gondor, not the rightful heir to another kingdom that doesn't exist anymore but had authority over the other kingdom that does. This way they can avoid having to explain in detail the history of the dunedain and the internal politics of Gondor, and why the claim wasn't previously accepted, but was with Aragorn.
__________________
His sword was long his lance was keen
His shining helm afar was seen
The countless stars of heavens field
Were mirrored in his silver shield
|