You answered your question in your first line.
I have to agree here, and this is the same reason that Debbie Stabenow did so well in 2012. Despite having a decent amount of name recognition, both Hoekstra and Land came stumbling out of the gate in their Senatorial campaigns. Combined with the fact that Stabenow and Peters ran strong campaigns, and the end result isn't surprising.
Something else to keep in mind--the last time someone from West Michigan was elected to either the Governorship or the US Senate was 1946, when Arthur Vandenberg won re-election for the fourth time. The list of Senators since Vandenberg?
Moody (Detroit)
Potter (Lapeer/Cheboygan)
Hart (Detroit)
Riegle (Flint)
Abraham (Lansing)
Stabenow (Lansing)
Ferguson (Detroit)
McNamara (Detroit)
Griffin (Detroit/Traverse City)
Levin (Detroit)
Peters (Detroit)
As someone who has lived in several places in the Lower Peninsula my whole life, I can tell you anecdotally that West Michigan is culturally secluded from much of the rest of the state. Hoekstra's fiery brand of conservatism doesn't translate well to suburban areas in Oakland, Macomb, and Wayne counties that would otherwise consider sending a Republican to the Senate, and Terri Lynn Land's "I'm a mom!" shtick may have appealed to the Stepford Wives-types that live in Kentwood, Grandville, and Ada, but came across as tone-deaf and condescending to voter-rich populations in Metro Detroit, who didn't find her endearing.