Very often the second half of a through flight number operates even though the first half arrives late. In this case, DL512 MEX-DTW was operated with an A319 and the second half DTW-MSP was supposed to be operated with an MD88. My guess is that the DTW-MSP cxl was not related to the late arrival of the MEX-DTW flight.
This has happened to me.
Just another aspect of the total farce associated with "direct" flights that require a change of aircraft.
More disingenuous marketing BS.