Milwaukee itself, the city anyway is probably not listening to WISN all that much. But the market has a few things going on
The counties surrounding Milwaukee County, particularly Waukesha, Washington and Ozaukee County are the probably the most Conservative counties in the state. At least as far as population. They're nicknamed the "WOW Counties" and make up the majority of the Milwaukee radio market outside of Milwaukee County.
From what I heard, there are not that many people under 50 still listening to radio in the Milwaukee radio market. You might notice the other high rated stations in the market target people over the age of 50. Classic Rock, Classic Hits, even the limited signal Oldies station (WRXS) has been getting decent ratings. Thus the talk demo has a higher share. WISN and WTMJ always had high ratings. They're the largest AM signals in the state. WISN's ratings are going up as the market itself ages and obviously the current political climate. WISN's ratings really started to go up really high during the coronavirus, so the political climate has a lot to do with it. The hosts have little ethics as far as I am concerned (Jay Weber was suspended for comments I will not post here) but they know how to keep a Conservative audience outraged, entertained and thus tuned in. Mark Belling has done this for years. Charlie Sykes did it for many years on WTMJ until he backed away from it. It's an artform on their part and the listening area has a lot of potential listeners.
And yes, the audience doesn't care if it's on AM. WTMJ has an FM translator but they don't even mention it on the air. WTMJ is also not the political machine it used to be. A lot of their more Conservative listeners probably switched over to WISN.