There was no requirement to solicit volunteers, as this was not an IDB eligible situation, because of this exception" (§ 250.6 Exceptions to eligibility for denied boarding compensation.)
"The flight for which the passenger holds confirmed reserved space is unable to accommodate that passenger because of substitution of equipment of lesser capacity when required by operational or safety reasons"

I think the $3600 in vouchers was rather good compensation since all they were required to offer was to get you on the next flight with seat availability, and the expenses relating to that delay.
I think it would have been better customer service to ask for volunteers, and may have gotten them for less than $1200 vouchers each, but they were under no legal obligation to do so.