A real shame indeed.
On a probabilistic scale there were more chances that a scum team featuring Emilia would realize that killing off Mango was too unbeneficial. If a scum team featuring Emilia did realize and used WIFOM still killing off Mango, that would have meant actually using it (which has not occurred since Emilia was town) and that would have been risky as well. Imagine Emilia claiming off the bat what I'm explaining, or a scum mate, had her been scum. I mean maybe claiming the WIFOM later on, sure, but overall my original point stands. More chances scum featuring Emilia would realize.
And either way, even if a scum team not featuring Emilia realized that, at the end of the day, it happens that no one of the town got to understand that in time. So either way the strategy to kill off Mango ended up victorious.