First, we have an open primary. Since Michigan appeared to be "solid Clinton" for the Democrats, many Clinton supporters likely voted in the GOP primary in order to derail Trump. I know one person myself who did this. Other Clinton supporters may well have stayed home. I only went because I feel strongly about the act of voting itself (and I voted for Sanders, but I'm supporting Clinton at this point). So we can't make any assumptions from the fact that more people voted in the Republican primary than in the Democratic primary.
Second, we have a businessman governor right now, and he's hugely unpopular with the Democrats. There have been frequent comparisons of Snyder and Trump, including at least one nationally syndicated editorial cartoon. Democrats will be motivated to make sure Trump doesn't do to the country what Snyder has done to the state.
Third, Snyder remains popular with his base. At least twice, when I've compared Snyder to Trump, a Conservative has told me it's not a fair comparison because Snyder actually knows what he's doing. And Snyder has actively refused to endorse Trump several times so far, saying he has too much to do at the state level to get involved with Presidential politics. (http://www.detroitnews.com/story/news/politics/2016/07/21/snyder-pence-trump-endorsement/87399002/)
So Trump will not have it easy in Michigan.