Why not? Because like most elections in the US, the primaries in Michigan are first-past-the-post, and a plurality suffices for victory. Only in a few states have laws or constitutional provisions requiring a run-off if no candidate gets a majority been adopted, and I am not aware of any state parties imposing such a requirement without a state-imposed requirement.
We do in Texas, as proof of the recent Paxton-Cornyn runoff.
Update:
Primary Runoff States
State Majority Requirement Notes
Alabama 50%+1 Both parties require a runoff if no majority is achieved.
Arkansas 50%+1 Runoff required for both parties.
Georgia 50%+1 Applies to both primaries and general elections.
Mississippi 50%+1 Runoff required for statewide offices.
North Carolina 30% Runoff is optional and must be requested by the second-place candidate.
Oklahoma 50%+1 Both parties require a runoff if no majority is achieved.
South Carolina 50%+1 Both parties require a runoff if no majority is achieved.
South Dakota 35% Runoff required if no candidate wins more than 35% in a race with three or more candidates.
Texas 50%+1 Both parties require a runoff if no majority is achieved.
9 states.... Thought it was more than that though.
