Its very possible they didn't do a good job selling the bump.
At the same time, this would be a harder deal to sell volunteers:
1) It was a Sunday night. Many passengers may have needed to work the next morning;
2) The flight offered for rebooking would result in a 22 hour delay. That's a long time.
3) Some passengers may not be frequent fliers, and had no need to travel within the next year, and thus no need for vouchers. Some business travellers may be prohibited by their companies from volunteering/keeping compensation.
4) Passengers have very little confidence that airlines aren't going to screw them over, so they may have viewed vouchers with suspicion, thinking that they would have a lot of terms and conditions associated with them.
Plus, it wasn't a massive aircraft, so you weren't dealing with hundreds of potential volunteers.