It is because a lot of republicans are stupid, and easy to manipulate. McCaskill put million behind Todd Akin in the GOP primary because she knew low information voters would pick the worst candidate. Greitens in a classic sociopath - just interested in himself and power. People forget he was a democrat. He didn’t have any awakening other than it is easier to win in MO as a republican.
https://www.kansascity.com/opinion/readers-opinion/guest-commentary/article262771943.html
One semester while I was teaching at Washington University in St Louis, he asked me to help promote his own course to students, and soon thereafter, we bumped into each other on campus. He thanked me for “fighting for all the right things” in the Missouri Senate, where as a liberal St. Louis Democrat, I filibustered bills that would have restricted abortion, made English the state’s official language and prohibited undocumented college students from attending Missouri public universities.
Former Missouri Democratic Party Chairman Stephen Webber had a similar experience. When Webber initially ran for state representative in 2008 — the same year when Greitens and I taught university courses — he considered Greitens a friend and supporter with whom he often shared meals and conversation about public service. Greitens attended Webber’s campaign kickoff and offered to serve as a surrogate speaker. In 2017, shortly after Greitens became governor, Webber recalled Greitens’ past support for a more progressive America, and this past week, Webber told me, “I always remember that he was a really big Obama fan.”
Not only did Greitens drive to the 2008 Democratic National Convention in Denver with former Gov. Bob Holden to witness Obama’s historic nomination, but he appeared to come away inspired: In early 2009, he began considering a bid for Congress against one of two Republican incumbents: U.S. Rep. Todd Akin in west St. Louis County or U.S. Rep. Blaine Luetkemeyer in central and northeast Missouri. He first approached Missouri Democratic Party officials, and then traveled to Washington, D.C., to meet with House Speaker Nancy Pelosi’s top lieutenant at the Democratic Congressional Campaign Committee, Rep. Steve Israel, and top DCCC staff. Ultimately, with 2010 looking like a very rough cycle for Democrats, he declined to run.
As the state of Missouri continued its rightward shift over the next few years, Greitens apparently surveyed the landscape and decided to switch parties
Read more at: https://www.kansascity.com/opinion/readers-opinion/guest-commentary/article262771943.html#storylink=cpy
That's precisely what he did. He's the most non-genuine candidate in the race - not to mention the other baggage he carries and had to resign as governor in disgrace.