I think it has to do with both simplifying the governance process and saving money.
The NCHA consists of schools from five different conferences plus an independent, so getting everyone and their conferences on board with decisions is cumbersome at best. Of course, it also gives the WIAC more control of the decisions.
Cost savings would come in two forms: annual conference fees and travel. The NCHA has 18 teams (7 men + 11 women), and I assume each pays the conference an equal annual fee. There are nine UW teams in the NCHA (5 men + 4 women), so the UW teams are paying half the annual fees of the NCHA - mostly for things that duplicate what the WIAC already has in place as an all-sports conference (i.e., commissioner, publicity director, administrative staff, etc.). In effect, the UW schools are already paying annual fees to the WIAC, so the hockey fees can be eliminated. Furthermore, if CSS and SNC join WIAC as associates for hockey, presumably they will pay WIAC fees for this, which can then be used to fill the one position the WIAC will need to add - coordinator of hockey officials.
Since WIAC men's hockey would have the same seven teams as NCHA men's hockey, any travel savings would be on the women's side - primarily by eliminating the long trips to Adrian, Lake Forest, Concordia, etc. Without the long trips, Stout may be willing to promote their women's club team to varsity, giving the women the exact same conference footprint as the men. (I'm completely speculating about Stout's women's club team, but I note they had a very successful season.)