Because you can't use the a studio for 2 things at the same time.
Overall, TEN's programs rate lower than Seven and Nine's. WIN News is actually one of the best rating show on WIN.
I think that's very much why Monday saw such a ratings shift. There is no other way to explain viewers moving from 9 News Sydney to WIN News Canberra instead of watching 9 News Canberra.
These viewers are obviously no all that interested in local news, nor would they be familiar with WIN News.
I think given WIN's long time association with Nine, some parts of the market are very confused about the difference between Nine and WIN. The change in programs would also not mean much to viewers who aren't familiar with the Ten Network, especially since Ten is no longer part of the market. I think it would be a common perception that Nine and WIN are both run by the same company, and they now just also show TEN Network programs - particularly to viewers who were used to affiliates showing programs from multiple networks before aggregation.
As a result, the difference between 9 Local News and WIN News may not be all that obvious, especially with very similar branding being used. It will take time for viewers to naturally find there they are different products and they prefer one over the other. This is where it becomes obvious that at least in the short term WIN have been incredibly smart keeping their name and the same news branding.
I wonder what avenues exist for Nine to stop WIN using the branding. Under normal circumstances WIN could be in a lot of trouble for copying Nine's branding, but Nine have obviously at some point given WIN permission to copy it for their own product, and presumably it was formally agreed that WIN could no longer use it once they stopped being affiliates (which would have been an easy omission to make years ago when there were no signs affiliation would ever change). I think both parties could equally claim that it is a significant part of their on air branding, and the longer Nine don't challenge it, the less claim they have against WIN.