Why would some customers, but not all, take advantage of mail-in rebates?

Stores sometimes offer “mail-in rebates” to customers who purchase a certain good to get a portion of the purchase price refunded. Typically, a mail-in rebate requires proof of purchase (like a UPC from the actual product along with a store receipt) and a completed form to be mailed in for processing, with the rebate being mailed in the form of a check or a prepaid debit card six to eight weeks later. Why would some customers, but not all, take advantage of mail-in rebates?