Unique promo codes have to be generated somehow. Integrating that generation into probably outsourced survey mechanism is likely not exactly trivial and may (if not done right) open ways for anyone to generate any amount of promo codes they want.

This is one kind of problems you get when you start separating different functionality into separate applications that don't known about other parts.

The above mentioned approach is significantly simpler to implement and easier to measure (when it works! ha) than uniquely generated promo codes. So you're also looking at a cost/benefit trade off.