Considering how WI-01 isn't really that Republican, its amazing how much Ryan over performs (with the exception of '12)
Lack of real opponents, a large fundraising base, and an ability to claim massive influence (moreso than almost anyone else in Congress) helps. If his district were to lose it's section of Waukesha, THEN there might be a race.
Losing Waukesha would probably cost Ryan the seat. Having that section in the district is what keeps any legitimate Dem from running against him.
It actually doesn't make that much of a difference. Paul Ryan wins 52-46 instead of 55-43 in 2012, and 60-40 instead of 63-37 in 2014. He generally over performs everywhere. In 2014, he got 79% in the Waukesha portion of the district, 68% in the Milwaukee portion, 55% out of Kenosha, 59% out of Racine, and 50% out of Rock.