I have no better answer than simply, because there are 12 inches in a foot.
By definition a conversion factor is one unit being used to express a group or multiple of another unit. Just like how there are 12 of anything in one dozen. 100 years in one century. 365 days in one year. 100 centimeters in a meter. 4 quarts in one gallon. 10 dimes in one dollar. They are equal to each other and, thus, can be used as conversion factors.