I do wonder if the option would have been doing better if the title was "head of city sewer jacks" or something.
Honestly I think it is a matter of prestige and suspicion.Yeah exactly. A martial advisor or a special council position as "Special Advisor" or something would work better.
We are a Grey Lady Magister if we are appointed to something public people are going to notice and speculate what is up. Appoint us as a Spymaster? Nobody bats an eye it is the expected position for a Grey Wizard, specific targets might be in question but nothing suspicious about us looking into one of the many terrible things in Warhammer people know lurk in shadows. Appoint us to "Head of city sewer jacks"? Everyone who doesn't know about Skaven is going to be wonder what we are actually doing because a Lady Magister obvious is doing that. Similar with special advisor people would wonder what it is we are fighting that a Grey Lady Magister is being used as a martial advisor.
Plus being Spymaster of Wissenland has a bit more perks to it as a full council position then just a side role. It might be assumed not offering us the full position would be either insulting or unattractive when we have other options. Which ... well we do! You want to make you offer as attractive as possible when you are trying to recruit top talent like this.