Because the community has additional experience and may have thoughts about a proposal. The impression one gets when moderating something can be very different from the impression one gets in the general case. Discussing such issues in advance helps prevent severe unintended consequences from occurring.
Very well, then: why should specific, large scale changes be discussed?
I’m intentionally ignoring the implication that this specific change was a “large scale” one.
Because a community is made up of its users, and if people find the changes negative enough, they will stop using the site.
Because the community has additional experience and may have thoughts about a proposal. The impression one gets when moderating something can be very different from the impression one gets in the general case. Discussing such issues in advance helps prevent severe unintended consequences from occurring.
In short, you’re hoping for the positive part of WWIC, while hoping the negative half doesn’t happen.
See references therein for applications to social websites.