I don't know. Is DeVos the Amway guy? Maybe the voters had a problem with the sincerity of a megabucks CEO touting a populist, I'm-for-the-little-guy message? The voters in WA State had a similar choice, sticking with Maria NeerDoWell against an big-bucks, job-cutting ex-CEO of an insurance company. The ex-CEO got slaughtered. My guess is the voters weren't too impressed running a megabucks CEO who might make some claims about being on their side.
To be fair, this kind of thing cuts across party lines. Maria Cantwell herself is an ex-CEO. You've got John Edwards moving into a $6M, 29,000 sq.ft. home and still mouthing off about their being "two Americas". Well, I know which "America" he is in.