Why do employees' families are not allowed to take part on the company's promo programs?

Say the company is Apple. There's a promo where they are giving away 10 iPhone 6 and 5 iPhone 6 Plus for 16 lucky winners (each receive either a 6 or a 6+)

Because if the family won it would look like it was fixed.

Prevent fraud

staff normally receive discounts which balances out

nothing stopping you from using family with a different surname eg cousins, in-laws