If look at racial attributes, Imperial is like normal human with nothing so great about them. Thy have no resistence toward magic like Breton, they have no advantage being mages like High Elves, they are not strong like the Orc and Nord, their combat prowess is moderate, they are not as skillful fighter as Red Guard. Their greater power "Voice of the Emperor" is just increasing people disposition once a day, Charm spell can do better, "Star of the West" just absorb 100 fatigue once a day, many Restoration spells can do better. Many other races have better greater power and racial advantages

So i wonder why they become a superior race and become "The Imperial", rule all other races that are more powerful than them?

Because they are a general jack of all trades race that is good at organizing. Not great strengths, but no great weaknesses either.

Several other races might be too self centered or "barbaric" to form an empire. Bosmer are too rural (ie wood elves) to focus on building an empire.

Altmers are an exception, they could develop an empire -- and so look at where they are in Skyrim.

So Imperials and Altmers seem the natural races for empires.

They have higher starting Personality than any other race. That's not as important in Oblivion (unfortunately), but in Morrowind it gave an Imperial a definite advantage in "getting what he wanted." If you play an Imperial in Morrowind, you get a sense of how powerful skills like persuasion and taunting can be.

Their racial advantages suit them rather well for the Knight class-type.