I don't know very much about Wisconsin, but I've heard the Milwaukee suburbs are redder than many rural parts of Wisconsin. Is this true? If so, what is the reason for this?
Yes, it's true and they have fairly consistently been the three most heavily Republican counties in the state of Wisconsin for decades now (Waukesha, Ozaukee, and Washington Counties).
In a post-Trump world, they are no longer No. 1, 2, and 3, but are still very Republican counties and more conservative than rural Wisconsin on the whole.
The reason for this is up for debate, although common explanations are:
1) The class-based voting preferences of manufacturing workers with a suburban managerial class
2) The prevalence of talk radio in the MKE suburbs
3) The prevalence of German ancestry
4) White flight
5) Being fairly religious, except Catholic and conservative Lutherans rather than more stereotypical Evangelicals
The answer is probably some combination of the above.