Because Imperials play a crucial part in the story now. Bretons don't really anymore. Imperials were added because they needed a face for the dominant ruling force. Could you really make a TES game now without Imperials? How would you explain it? Bretons on the other hand, they would just be like a fart in the wind.
Yes, yes you could make a TES game without the Imperials. The imperials are a race of nobodies. Actually, they're Nords, just a different Nordic Culture.
Before
Morrowind, the Imperial Race was distinguished by Culture, not Physical Appearance. Everyone in
Oblivion is an Imperial. Most happen to be Nords. Some Imperials were kinda fuzzy and spoke in third person. A few were green and red lizards. But they were all Imperials.
Nords come in two distinct subraces: The Nords of Skyrim, and the Nords of Cyrodiil.
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Morrowind and its [censored] of the series lore.