Here's why:
* "Proudest" is the superlative form of the adjective "proud." It's used to compare more than two things or to indicate the highest degree of pride.
Example: "She was the proudest mother in the world."
* "Most proud" is grammatically correct but less common. It's used when you want to emphasize the intensity of pride, but it's not the most typical way to express the superlative.
Example: "She was most proud of her children's achievements."
While both are technically correct, "proudest" is the preferred and more common choice for the superlative form of "proud."