Here's why:
* Possessive nouns show ownership or belonging. Examples include: "my", "your", "his", "her", "its", "our", "their".
* "Both" is a determiner that refers to two things or people. It is used to indicate that something applies to two items equally.
Let's look at an example:
* Correct: "Both of the cars are red." ("Both" is a determiner referring to two cars.)
* Incorrect: "Both's color is red." (This would imply "both" owns something, which is not correct.)
To make a possessive phrase with "both," you would need to use a possessive pronoun:
* "Both of their cars are red." (Here, "their" shows possession.)