Here's why:
* "Who" is a relative pronoun used to refer to people.
* "That" is a relative pronoun that can refer to both people and things.
While "that" is technically correct in some cases, using "who" is considered more formal and elegant when referring to people.
Example:
* Correct: "All who attend the conference will receive a certificate."
* Possible, but less formal: "All that attend the conference will receive a certificate."
Note: If you are referring to something other than people, "that" would be the correct pronoun.