When we're designing anything, we usually fall in love with our best solution. We get attached to how well we've resolved the issues, made the tradeoffs and combined some partial solutions into our favored design. We embrace the assumption that we will end up with the best outcome if we stop messing around with what we've come up with at last. We're usually afraid that will throw out the baby with the bath water if we change the design. Considering an added criteria can be very upsetting for these reasons.
Added criteria usually make a design better. It becomes more responsive to it's larger context. It make accommodate more stakeholders It may deal with long term issues in a better way. It can possibly serve the interests of customers/patients/members/students with less expense. Added criteria can even reveal some creative options that had not been considered thus far.
Choosing to reopen the design process can polarize the participants. Some may regard it as a threat as I explored above. Others may see it as an opportunity to do a better job. It depends on where each person sees her or himself. Anyone on the ascending side of life cycle curve will see progress ahead and no place to go but up. Those on the cusp of the curve will be wary of losing ground, entering a slippery slope and going past the point of diminishing returns. Those on the downside will be seeing everything going from bad to worse, catastrophizing about the future and foretelling a series of unfortunate events.
When there is resistance to making a design more responsive, responsible, respectful and resourceful, consider where people are at in their lives. It may not the added criteria that's at fault.