So have the most prominent politicians who could have run.
Wisconsin State Assembly Majority Leader Robin Vos and former Republican Party National Chairman Reince Priebus were the only potential candidates with name recognition outside their own respective bailiwicks, but withdrew from consideration in the first couple days after Ryan announced his retirement. State Senators David Craig, Van Wangaard, and Steve Nass and Assembly members Amy Laudenbeck and Tyler August also took themselves out of the running.
So far, the only announced Republican candidate is Paul Nehlen*. Nehlen had announced his candidacy well before Ryan withdrew his; Nehlen had challenged Ryan in 2016, representing the Breitbart/Steve Bannon/Ann Coulter wing of the party. He was endorsed by Phyllis Schlaffly, former Congressman Tom Tancredo, and Gun Rights PAC that year. Donald Trump had toyed with the prospect of endorsing Nehlen in retaliation for Ryan's reluctance to support Trump's presidential campaign, but ultimately made nice with the Speaker of the House, who trounced Nehlen in the 2016 primary by a 6-to-1 margin.
Newspapers in Wisconsin's First CD were calling on Nehlen to quit the 2018 race because of his execrable racist views, even before Ryan pulled out. He was banned from Twitter in February after posting an image of Prince Harry's biracial fiancée Meghan Markle that was photoshopped to make her look like prehistoric Cheddar Man. A few days before that, he had posted a list of his critics, their phone numbers, e-mails and Twitter handles, adding, “Of those 81 people, 74 are Jews, while only 7 are non-Jews,” sparking a neo-Nazi flame war against them.
In another anti-Semitic tweet, Nehlen posted “Poop, incest, and pedophilia. Why are those common themes repeated so often with Jews?” But he's an equal opportunity bigot. He has told listeners of fellow Nazi David Duke's podcast that he'd like to see Trump's Ugly Wall equipped with “armed machine gun turrets every 300 yards. And you can automate those. Anyone who approaches that barrier will be treated as an enemy combatant. Man, woman or child.”
Bryan Steil, a former Ryan staffer currently serving on the board of regents of the University of Wisconsin, and State Rep. Samantha Kerkman are still weighing their options. Lack of name recognition at this date shouldn't hurt either one; very few residents had heard of Paul Ryan before he first ran for Congress in 1998, but he had the strong backing of the national GOP. Redistricting in 2000 and 2010 coupled with exurban sprawl out of Chicago has made the 1st CD increasingly Republican, so whoever wins the August primary may still have an advantage over the Democratic nominee even in this "blue wave" year.
* Update: There's some guy named Nick Polce who also announced his candidacy for the Republican nomination last fall. All I know about him is that he's an Army Green Beret veteran (2002-2014) from Lake Geneva who checks all the far right-wing boxes and describes himself as part of the "freedom movement."