Middenheim is the power base of Ulric instead, where they view Sigmar (who was an Ulrican) as a powerful historical warlord and founder of the Empire, rather than as a god.

The largest temple of Sigmar is in Altdorf, where the cult of Sigmar is based, and the two cults contest each other for power politically (although the Sigmarites are far more powerful these days, with the Grand Theogonist effectively being the Empire's religious leader).

Middenheim is ridiculously defensible, however, and it's central position means it's hard to ignore, so most serious attempts to take the Empire would have to deal with it at some point! That might be why most chaos/skaven attempts both aim for it and tend to grind to a halt there.

I take your point on the chaos incursions being the driving focus of warhammer's plot historically, and I suspect that just having three armies dedicated to it was a good enough basic plot point to get as many players involved with the global campaigns of old as possible. Perhaps they might bring Nagash out of retirement too at some point!