Plenty of them do. You send them 50 low payments of $10, and upon receipt of the 50th payment, they send you the scanner!
Seriously, look at it from the retailer's point of view. There's so little markup in scanners these days that if you offered them on a "rent-to-own" basis it would be easy for one deadbeat to kill any profit margin you might have had. No one's going to take that sort of a risk. Besides, if you qualified for dell credit, you probably also qualify for a Visa, Mastercard or (shameless plug for my current employer

) Amex card, and then you can make payments on it that way.
-AZ